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Energy Efficiency - The big picture

For some time, economic policy discussions have been focused on energy. The prosperity of our modern industrialised societies in the first place has only been possible due to the availability of large amounts of cheap energy: long distance travel, warm homes, diverse commodities, fast communication - much of this is unconceivable without a functioning energy supply. But at the same time the huge crises of our times also revolved around energy - more precisely, around energy sources and access to these.

However, this is only one aspect of the multifaceted concept of energy; one that is often very much in focus nowadays. However, we easily forget that energy is a quantity that was originally elaborated and developed in the natural sciences1) , the core significance of which is quite far removed from the “sales item energy” that is predominantly perceived today. This article will deal with the relationship between both, and the insights gained in the process will allow us to avoid the perpetual cycles of energy crises in the future.

For the whole lecture, you can also see the video

In physics, “energy” plays an even more significant role; it could even be said that physics is the “science of energy”. Let us set out to learn how the energy concepts in physics and in the economy relate. This will allow the topic to become accessible in an interesting and different way.

Figure 1 In economics and physics, energy is considered quite differently. This presentation will lead to a deeper understanding of the common core and will give us hints on how to solve the related problems.

Coffee - nice and hot, please!

Who doesn't like hot coffee? The problem is that coffee in an ordinary pot doesn't stay hot for very long because it loses heat through the surfaces towards the colder surroundings. Clever engineers calculated the “heating demand” and let the lost energy be supplied back by means of an electric hotplate; that's the active method. But this can also be done without a lot of energy: If we fill the coffee in a very well-insulated container (a thermos flask), then the heat loss will be so small that the coffee can be enjoyed hot even after several hours. It may be surprising that for almost all services today our energy system is needed solely to compensate for losses. This exactly is what our presentation will be about.

Figure 2

In modern industrialised society, “energy” plays a key role for the entire functioning of the economy. Leading economists have described “energy” as the lubricant of the national economy and regard “cheap energy” as the basic prerequisite for prosperity and growth. Almost the whole of society has a similar view - including the harshest critics of “economic growth”. This perception is strongly influenced by our day-to-day experiences: energy is mainly involved in the form of energy carriers (such as oil, gas and electricity) and as such must be paid for. There is an entire energy sector which must ensure that sufficient and cheap energy is always available. End users consume these energy carriers, of which none is left in the end. This results in prosperity.

Figure 3 Energy carriers are the central concept in an industrial society. All prosperity is based on the supply of sufficient and inexpensive energy carriers.

Physics can be regarded as the “science of energy” 2). The concept of energy was first elaborated in total clarity in the 19th century. We will describe the main features of the concept here in its simplest form. It turns out that the main characterisation of energy processes in daily life is their “efficiency”, which interestingly is a term economists like to apply to the use of human activity but are keen to ignore in relation to energy 3). Once we understand this relationship, we will be ready for a fresh approach: Energy as an instrument in a knowledge-based society; this aptly reflects the facts, because in the same way as tools that are degraded “only very gradually”, energy consumption will also be very low.

Figure 4 In physics energy is a very basic concept. There are two fundamental theorems: the law of energy conservation (1st law) and the law of entropy (2nd law). Energy-efficiency turns out to be the key to understanding processes in our daily life

If we want to explain the concept of “energy” in colloquial terms, then “the ability to implement changes” would be the best way to put it. Visible, perceptible, generally discernible changes in the systems are associated with energy flows (the transfer of energy from one system to another or the transformation of one form of energy into another form). What is difficult to comprehend for many people is the fact that with such transformations and transfers, energy is not “consumed” at all, rather it still exists but “only” in another form or in some other place.

Figure 5 Energy can be seen as the potential to change things in our world

In extreme cases, such changes may be dramatic. If we transform an extremely large amount of energy in a very short time, we refer to this as an explosion. This changes very fundamentally many things around it - and that is the key reason why the military is interested in energy and especially in systems with a high energy density. Many physicists and engineers are fascinated by this aspect, and it is also lucrative from the economic perspective. However, except for New Year celebrations, these things hardly play a role in our daily life - when they do, it is usually in a very unpleasant way.

Figure 6

Carrying a piano to the second floor

Let us start with the most elementary experience. Before us we have the task of transporting a piano to the second floor. For this, we must carry its weight (this requires the use of force by us), and maintaining this force while we lift the piano precisely in the direction of the applied force “up” through the staircase. “Force times distance” - that is the work performed by us in the process: more work if the item has a larger mass, and likewise more work if it has to be carried to a higher level. This has been calculated in the illustration shown here. Actually, with this process we have already understood the entire concept of energy: the work performed in this case is not lost but is instead “stored” in the potential energy of the piano. Anyone who doesn't believe this, can just let the piano fall down the staircase; even after many decades, the effect will always be the same.

Figure 7 The basic concept of potential energy: it's stored 'in the piano' (more precise: in the changed position of the piano in Earth's gravitational field), after work has been done to lift it to the 2nd floor. It's very easy to calculate that energy. Now is that 'a little bit' or is it “a lot of” energy4)?

For another completely different comparison, here is a typical modern use of energy: we are heating a building. For an average house in the existing German building stock, around 3000 litres of heating oil are still required for this today. How much energy does this fuel contain? It is easy to memorize the calorific value of “heating oil” to be almost exactly 10 kWh/litre. This introduces the most common energy unit for daily use: the kilowatt hour kWh. Oil, natural gas, electricity, district heat etc. are charged by using this unit. Many people therefore have an idea of the scale of 1 kWh. We can convert this unit into the SI base unit “joule” (third line in the slide5)) . The annual heating oil consumption for heating this house is 3000 times 10 kWh, or in SI base units 108 billion (!!) joules. That is 26.2 million times more than our example of the “carrying a piano to the second floor”. What is particularly clear from this is that the scope of the energy turnovers we are accustomed to is huge, much higher than the body's performance that is naturally possible for us.

Figure 8

Law of energy conservation

So far, we have specifically learnt about three forms of energy here: mechanical potential energy, chemical energy and heat. The table on the right shows an overview of the forms of energy dealt with in physics today. The brilliant thing about the concept of energy is that all these forms of energy can be converted into one another. As an exercise, a matrix can be prepared which shows which kind of process (or “machine”) is often used for converting one of these energy forms into the other form. For example: we use a light bulb in order to convert electrical energy into electromagnetic energy (i.e. light). The characteristic formula of the respective energy form is given in the second column of this table. We already know about Row 1 and Row 10, the reader does not need to bother with the others for the time being; if anyone is interested, a whole lot more is explained in our "Building physics course", for example.

Figure 9

The conversion of mechanical potential energy (energy of position) into kinetic energy when an object falls from a height is familiar to us from everyday life. The conversion of small differences in height into identical small 'kinetic differences' in is easy to comprehend - we have explained this in more detail here: Law of (mechanical) energy conservation. In this overview we will only state the well-known result: the “formula” for classic 'kinetic energy'.

$ E_{kin}=\frac{1}{2} m v^2$.

Figure 10

“Energy is not lost”- at least not in a closed system. That is a system in which NOTHING can cross the system boundary (neither matter, nor any kind of effect). The energy remains inside such a closed system6) . Sometimes we hear the statement by some VIPS (“Very Important Physicists”) that “there are no energy losses” due to the law of energy conservation. That is nonsense: this would only be true if all systems were strictly closed systems - energy flows which otherwise cross the boundaries of a system towards the outside can rightly be described as “energy losses”, these are lost to the system. (You can throw your money out of the window. While the money may not be ‘lost’, it’s certainly a loss for you).

Figure 11

Open systems are the rule: Here 7) also, the system must be clearly characterised by a spatially delimited system envelope, except that this can now be crossed by energy and mass flows. The advantage of the concept of energy is also apparent here because it is now obviously enough to balance the energy “crossing the boundary” through this envelope. If such an energy flow is negative (seen from the perspective of the system) then by definition this is a loss, while if it is positive, it is energy coming in from the outside. The energy balance is now obvious 8): the change in the energy content (the “internal energy”) is the same as the sum of all energy flows passing through the envelope; or in other words, the difference of the energy gains and the energy losses.

Figure 12

The energy flow diagram is a popular technical illustration. Here the emphasis is on the energy flows passing through the system: as a rule, “incoming energy flows” come from the left and energy flows leaving the system are on the right. Often, a distinction is made between “loss flows” and “useful energy flows”, where the loss flows are often depicted as an arrow pointing upwards “into nirvana”. Because most technical systems are in a steady state9), the energy in the system is usually constant i.e. the change in the internal energy is zero. Energy balancing will then be quite easy: the total of all gains is the same as the total of all the losses. The efficiency of a system can then be defined: This is exactly the proportion of the delivered energy flows $E_{in}$ ,to the transformed useful energy flow $E_{out}$ :

${\displaystyle \eta = \frac{E_{out}}{E_{in}}}$

Figure 13

Frequently, there are entire chains of consecutive energy systems in which the respective subsequent system processes the useable output of the preceding system. There are related losses in each link of the chain and an individual efficiency can be given for each individual system. An “overall efficiency” results for the entire chain as the product of the individual efficiencies. There are also more complex flow diagrams with several output flows (often called “coupled or combined products”) and “loops”, which are created if flows from a subsequent process are fed back 10) into a preceding system 11) . These so-called Sankey-diagrams are a powerful and at the same time highly illustrative tool for balancing of complex energy systems on a general level. Engineers and energy suppliers usually let these energy flow diagrams end after the first machine of the energy user (“end” user or end consumer or client12)) . What comes out of this system is often described as “useful energy”. However, because this energy usually isn't stored by the end user, and the energy flow doesn't end here either, strictly speaking the description “useful” energy is misleading. As physicists and engineers, we will follow these energy flows a few steps further, namely until they have actually left the last system used by humans13) OR “finally” remain in a system 14).

Figure 14

The last system that is in use is shown on its own here, in the case that the generated benefit itself cannot be measured in energy units 15) . This is actually the most important sub-system in the chain: this is where the energy service is produced, which is what the entire chain is ultimately being used for: the actual benefit of the energy use16) . Here also, the output is an energy flow. However, this is only a loss, because after this utilisation the energy is “finally used up” from our perspective: it leaves the last considered system and it passes through into the environment 17)).

Figure 15

Caution: here it may seem that the entire energy application in general is only a “meaningless process of disposal” which we could do completely without as well. But that is not the case: in order to provide the service, the relevant amount of energy is often really needed on account of specific boundary conditions 18) . However, very often these boundary conditions can be changed with modern technology so that the total extent of the energy flows required for the service becomes much smaller than usual. That is what we will now deal with using key examples.

Energy Service: Space Heating

We like to have comfortably warm living areas - which cannot necessarily always be said of the natural environment, where temperatures prevail that are noticeably below the comfortable limit, e.g. in Central Europe between October and the beginning of March. We therefore create comfortable conditions inside our buildings; usually we do that by means of a system which is called “heating”. We therefore want to call this energy service “space heating” here. This consists of keeping the temperatures in the occupied area at a comfortable level (between 20 and 24°C), even if it is colder outside, and especially then. How much colder this is, is quite relevant for the energy expenditure, which is why it is ultimately the temperature difference from the outside which characterises the extent of the energy service.

Figure 16

It is easy to see that the energy service increases with the heated area and also increases with the length of the time interval in which it is provided. The quantified value energy service: space heating thus results in a temperature difference times the useable area times the time period, which leads to the unit Khm². And although this energy service now almost looks like “a form of energy”, it is not; instead, it is an indication of the state 19), the characterisation of a non-equilibrium state, which we make into a steady-state by means of passive and active systems. How much energy is required for maintaining this steady-state depends mainly on how much heat we allow to leave the final use system ‘building’ again20) . The active part of heating consists solely of replacing these losses again. This becomes even clearer with the energy flow diagram in the following slide. A note on the importance of this would be appropriate here: around 27% of all final energy consumption in Germany is used for space heating. This is the largest chunk after energy consumption in the transport sector (ca. 30%). It is therefore worthwhile to address this.

This is actually the crucial energy service system for “space heating”: It is the building itself on the inside of which (orange) the service is provided: indicated in a simplified way with a constant indoor temperature. A physicist immediately knows that at a constant indoor temperature the internal energy of this system does not change; but it will change if we simply leave the building to itself in colder surroundings: then the heat would flow outward into the surroundings. This heat ultimately leaves our utilisation chain and can only be retrieved from our environment with a huge effort 21). This is therefore lost energy; in fact these are exclusively energy losses, not a single kWh of the heating energy consumed in the previous winter a few months later “exists” anywhere inside the building. The amount of energy losses is determined by the quality of the envelope surface components of the building: if these allow rapid heat transfer (e.g. single-glazing), then the losses will be high; if this is vacuum glazing then the losses will be small.

Figure 17

The extent of the losses can even be calculated based on the laws of heat transfer: Materials such as concrete conduct heat very well from areas of higher temperature into areas with a lower temperature. If the building envelope consists essentially only of concrete, then the heat losses will be very high and accordingly a lot of additional heating will be necessary. Many of our existing buildings are built as if it did not matter at all that huge amounts of energy can just dissipate into the environment.

We see here that it is first and foremost the quality of the energy service system, in our case characterised by the thermally insulating effect of the building envelope, which determines the 'demand' for the energy that has to be provided.

Figure 18

Here is the quantitative summary of the energy losses for an actual building, like the ones usually built in the 1990s, for example 22) . Easily calculable losses are piled up on the right side: The ventilation, the exterior wall and the windows are conspicuous here. Some of the losses are practically provided free of charge due to solar gains through the windows and due to waste heat from persons and appliances, but because the losses must be fully 23) compensated, the most significant energy flow on the gains side is the heating energy (shown in red). In this case this is 96 kWh/(m²a)24) . Not as much as in most existing buildings, but still a huge amount of energy which, when supplied with oil, would need to burn approximately 1500 litres of heating oil every year.

Figure 19

The losses can be extensively reduced through technical improvements of the utilisation system. “Technical” here means structurally, e.g. better windows (triple glazed), better thermal protection in the roof (e.g. insulation material installed in the top floor ceiling), cladding of the façade. With state-of-the-art construction technology, these improvements can reduce the losses not just “a little bit” but rather to a very great extent: in the example shown here, this new construction was built with a building envelope which resulted in only a quarter of the usual losses. However, because the amount of “free heat” remains approximately the same, the reduced losses are almost completely absorbed by the heating energy which now amounts to just about a seventh, namely approximately 14 kWh/(m²a). Incidentally, this is such a little amount of heating energy that we no longer need an oil or gas based heating system for this. Such small amounts of energy can come from renewable energy sources without problem - and they can also be easily supplied via the existing power grid25).

Figure 20

In practice it’s looking like this: The recently applied thermal insulation consisting of “packed air” has a thermal conductivity which corresponds to just about one eightieth of that of concrete. The layer of insulation is about the same thickness as the load-bearing concrete wall. After insulation, the heat loss is reduced to an extremely low value. The insulating effect of the windows used here is also excellent26). The windows are correctly positioned in the middle of the insulation layer so that thermal bridges are also avoided. Such buildings lose only a little amount of heat and therefore need very little active heating; in this case heating and cooling can take place just using the homes ventilation system. In this building the energy service is provided mainly through efficiency techniques, that is, 'passively'27).

Figure 21

From this specific example, let us return to the fundamental considerations: we want to determine a scale for the efficiency of the utilisation system also if the relevant output parameter does not have the dimension of energy. Because the benefit (the energy service) is always greater than zero, but the expenditure may also be zero 28) , the conventional “efficiency approach” is not expedient here. Instead we use the reciprocal value, the “performance factor”, or in our case, the “specific energy expenditure” in kWh per unit of the energy service ‘useable heat delivered’ divided by the energy service. In this slide it has been calculated that this leads to the dimension $\frac{\mathsf{W}}{\mathsf{m^2K}}$ for the specific energy demand.

In the building sector a well-known unit!

This is a kind of summarised overall energy loss per m² of living area and Kelvin of temperature difference.

Figure 22

What are the essential technical measures for keeping the heat losses of buildings low? These are the three measures illustrated here:

  1. Thermal insulation (without gaps of course, ensuring that thermal bridges are avoided)
  2. Improved windows (primarily triple glazing instead of double glazing)
  3. Heat recovery from the extract air flow (because fresh air is needed but it is cold, so we preheat it using the heat from the used air flow)

Due to millions of built projects, today we know that all these techniques work as intended and that with their use a large number of other problems can also be solved at the same time.

Figure 23

And - that works even in the case of existing buildings. The picture shows an existing building on the left, which was in need of modernisation in any case. It was equipped (on the right) with improved thermal insulation from top to bottom, new bigger windows, balconies and even a heat recovery system. This had the effect of saving 85% (!!) of the space heating energy that was previously required, the apartments have become brighter and more comfortable, and the building substance has been preserved for a further few decades. Today, we call this approach"EnerPHit" and it has proved successful thousands of times in diverse buildings and in all kinds of places and climates worldwide.

Figure 24

If we look at the building again using a thermal imaging camera 29), the effect of these measures will also become visible from the outside: the façade is hardly any warmer than the tree standing in the open and even the windows are hardly radiating any heat. This is in strong contrast to the heat radiating from an uninsulated building in the background. By the way, this building is described in more detail in the EnerPHit-Example

Figure 25 EIFIS = Exterior insulation and finishing system

How relevant actually is space heating? The adjacent pie chart shows that the two energy services “transportation” and “space heating” each account for around 30% of the total final energy consumption. These are thus the most significant single energy applications. From the way in which we improve the efficiency of these uses, we can learn something for the other energy services as well, and the following slides will also illustrate this. The third largest chunk “process heat” is in fact a mishmash of completely different applications, from ore smelting to candle-making; experts for process technology can develop concepts for improving the efficiency of each of these processes - which in many cases has also been successful. The solutions are available, but to a huge part not used.

Figure 26

Energy service: Transport

Transportation is the other major reason for our high final energy demand; this energy consumption is largely due to motorised private transport, i.e. cars and motorcycles. A measure for the energy service has been in place in Europe for decades: the annual mileage of vehicles. In 2010 this was 905 billion kilometres for cars (= sES) (energy service) in total in Germany, 718 billion kWh of petrol/gasoline were consumed as the “input” for this purpose. For a distance of 100 km these vehicles therefore had a specific energy consumption of

$ e_{spez} = \frac {E_{sprit}}{s_{EDL}} = 80 \frac {kWh}{100 km} $

(Incidentally this equates to almost exactly 8 litres per 100 km; this value has hardly changed over the decades30) ). We will use this as a reference value here for further analysis.

Figure 27

The fact that this energy service (transporting a person from A to B) can certainly be provided with considerably higher efficiency also with a conventional car is shown in this slide. A mass of 1400 kg contains everything that is necessary for a good car - the car is also fast enough and with 45 kWh per 100 km, the specific consumption is only 56% of the total fleet average. Thus considerable increases in efficiency are also possible in car transportation and are even available with conventional vehicles on the market.

Figure 28

The breakthrough in efficiency came with the use of electrical propulsion systems - because modern electric motors in this class have efficiencies of more than 95%, which is much higher than any combustion engine31) . Added to this is the fact that electric motors can work reversibly: I can “brake” using the motor and this then works as a power generator and recovers kinetic energy for me which is otherwise normally converted into useless heat in the brake disc 32).

Figure 29

The The limits of improved efficiency aren't yet reached with this, even for vehicles with combustion engines. A large car manufacturer had already created a prototype for a “1-litre car” - it was mostly the engineers who showed what they could actually achieve. However, a vehicle of this quality then never has been introduced to market 33).

Figure 30

As practice just shown, the efficiency of vehicles can obviously be improved to a great extent. Is there a limit to this in physical or technical terms? An initial clue for answering this question can be found in bicycle technology: if we calculate the (complete!) food intake of the person riding the bike as the energy expenditure (input) of the system, then this will result as 2.4 kWh per 100 km. This is just barely about 3% of the reference consumption. A bicycle is a vehicle with an extremely high technical efficiency. This will apply even if we additionally equip the bike with an electric hub motor - and we will then become a little faster on average.

Naturally the energy service here is not comparable to one with that of a car: I am slower, I don't have a roof over my head and I can't transport as many things. But, this technology can be expanded in this direction without problem. See Slide 32.

Figure 31 Is there a limit to how efficient we can be?

The following video fits very nicely here, which shows the performance of a well-trained athlete on a bike: he manages a little more than one minute with a mechanical output of 700 watts 34) . 65 seconds for a professional racing cyclist, which is roughly 700 W x 65s = 45500 Ws = 12.6 Wh = 0.0126 kWh(!). Yes, that's certainly considerably more (almost ten times!) than our initial example “carrying a piano to the second floor” (Slide 7). What is apparent here is: services providing heat use up significantly more energy than we intuitively want to admit. However, 0.0126 kWh is still a tiny amount of energy compared to what we are accustomed to. Yes it's completely mind-blowing to see the gigantic amounts of energy our modern supply system constantly supplies. A typical 12 kW gas or oil fired system easily uses up 288 kWh on a single day in winter! That is about 23 thousand times the amount of energy converted in the video:

Olympic Cyclist Vs Toaster: Can he power it?

Racing professionals normally manage to reach speeds around 40 km/h on average with more like 350 W for a longer period of time. This is mechanical energy of around 0.9 kWh/(100 km) and matches the energy efficiency values of bicycles 35) .

And we can be even more efficient than with bicycles. As demonstrated by this car of the University of Bochum participating in a Solar Competition: a three-seater, quite fast with a speed of 100 km/h based on bicycle technology is even more efficient 36) . Such vehicles have a consumption of just a tenth of that of an average electric car today. Prototypes that have already been realised in practice show that there does not seem to be any lower limit (>0) for the reduction of energy losses in transportation. Now it’s time to look at this from the point of view of basic physics.

Figure 32

Moving fast?

What does physics say about this? We have already learnt about the definition of the energy service in transportation (person-km or cargo-km).

For the further analysis, we must first be clear that we humans travel along predominantly closed paths: At night I usually lie down in the same bed as yesterday. Okay, this is almost always true, at least after a couple of days 37) . Mathematicians are very familiar with such closed paths: These are used to characterise force fields - and in physics it is apparent that the Earth's gravitational field is an important force field in our normal environment. Now this has the property of being a “conservative force field”; this is characterised precisely by the fact that the sum 38) of force times distance (that's work!) is ZERO =0 along each closed path. For a change of location followed by returning Back to the start (even on a different path), in net terms no energy is “used up” in the gravitational field at all. That this is actually “correct” 39) is proven by the fact that for the last 4 billion years our planet Earth has been spinning in an essentially stable elliptical orbit around the Sun with an almost zero net energy consumption (year after year) 40) .
Then why does our “everyday sense” tell us that travelling along paths on Earth is a strenuous task? Ultimately this is again due solely to inefficiency of the system with which we provide the energy service of transportation. Yes, the planet doesn't exactly make it easy for us: Friction steals kinetic energy from us everywhere, whether it is friction of wheels on the road or air resistance in the airstream; but also the fact that so far, the regularly gained kinetic energy is not “recovered” during braking contributes to this inefficiency. In principle, all these loss paths can be reduced almost at will (e.g. through magnetic bearings and by reducing the atmospheric pressure in the surroundings of the vehicle; all this has also already been demonstrated successfully41)) . The facts become clear in a particularly impressive way when we pursue the question that was already known by Galileo: he carried out experiments on this and found approximate solutions to this: given two points A and B somewhere in the city. No forces are acting on these apart from gravitation 42) ; A is not deeper than B. Now which is the “fastest path from A to B” if the mass m is simply released in the gravitational field along the path? (This is the problem of the fastest curve of descent, known as 'Brachistochrone'). Every physics student knows about this task, it is used as a standard introductory example for the “calculus of variations”. Everyone remembers that precisely the intuitively guessed path “straight line through A and B” is not the fastest, instead the surprising answer is that a path that is longer, travels lower down the gravitational field and then ascends again to B is the fastest. The students (and usually also the professor) consider this result an amusing peculiarity and do not pause to think this through more thoroughly or even to ask about the practical consequences.

We will do that here at least for a specific case: for the cycling path from A to B (on the same level here). Both cyclists start at the same speed vx0 in direction x. Tom is in a hurry and follows the horizontal expressway on an embankment, while Anna duly follows the ups and downs of the service road marked as a cycle path (this is also asphalted smoothly); friction on this short section is negligible and none of the persons pedals or brakes. Who will be the first to reach the destination? Surprisingly, it is Anna, despite the detour, and it is even easy to see why.

Figure 33

We are looking at the bicycle on the path with dips. Its speed consists of a vertical (downward) and a horizontal component. If the body is below the reference level by the amount h, then its potential energy has decreased by mgh. Its kinetic energy has increased by exactly this amount, consequently its speed has increased. The vertical velocity component generally does not play a role with reference to the travel time 43) . However, the horizontal component has also increased everywhere along the entire stretch (in the lowest point all of the potential energy has converted into the kinetic energy of the exclusively horizontal velocity here). At every point this body is therefore faster than the cyclist on flat land.

Figure 34

If we consider the energy-related essence of the brachistochrone path we can see that this is by no means a peculiar exceptional phenomenon, but rather it is a fundamental principle according to which most natural processes44) take place. Yes, changes (in location in this case) are associated with energy flows; however, if you're smart you will just borrow the respective energy from somewhere and give it back again after the energy service has been rendered 45) . The actual energy service (taking the bike from A to B) namely needs no energy 46) ; in any case, it does not 'consume' any. The bike rider Anna borrows energy from the gravitational field for acceleration, and converts it into kinetic energy of her bike. On arrival in B, the energy has fulfilled its function, it is no longer needed and has been given back (in full quantity!) to the gravitational field.

Can we use this trick in practice? Certainly, that's all a vehicle with an electric motor/generator does: it 'borrows' the energy for acceleration from the battery at the start of the journey and in the end feeds it back in by means of the generator-brake.

Energy service: Light

Final energy use for 'artificial lighting'47) currently accounts for just 2.8% of the total consumption. “Light” is among the many important energy services provided with the use of energy today, the individual energy demand of which is not particularly noticeable – although, it all ‘adds up’. In spite of this, light is virtually a symbol of the progress which a reliable energy supply has brought us. For a long time, “light” was commonly synonymous for all applications of electricity. We will therefore also take a look at this field of application, and find that it is also a good example of the importance of efficiency in the area of energy applications. The natural light of the sun is not always enough for adequate illumination everywhere. In order to obtain a point of reference, we will assume 500 lux for illumination of workplaces that is normally stipulated; if the work area comprises 1.4 m², this luminous flux corresponds to 700 lm. The luminous flux may be used as a good characteristic parameter for the energy service. A special aspect here is that a lumen can in fact be identified directly by means of a useful energy flow 48) .

A candle - probably among the first artificial light sources; provides just 0.1 lm/W, the rest of the energy is directly converted into heat. With this, a reasonable level of illumination for a desk is not possible at all. The pressurized mantle lamp with 5 lm/W was 50 times more efficient - and could certainly illuminate a desk with a power of 140 Watt (efficiency of around 2%). The incandescent bulb was the next step forward in efficiency - for the first time, adequate light at night was also possible without fumes and the risk of fire in the room; 12 lm/W equate to an efficiency of 5%; the incandescent bulb still becomes so hot that a user can easily burn the fingers on it. But, electrical energy can be directly converted into light by stimulating specific quantum state transitions; the fluorescent lamp technology soon became available for this, which already achieve an efficiency of around 20%. The more conversion steps a technology is able to save, the more efficient lights are: although semiconductor recombination photons (“light emitting diodes LED”) have been known for a long time in principle, it was only in the past few decades that these developed into a light source that could be used on a broad scale. Efficiencies of commercially available lights range between 75% and 80% and the best systems already come close to 100%. Thus the electrical output necessary for good lighting has now dropped from 60 W to just 3.5 W. For a completely renewable energy supply system, this is no longer a problem anywhere in the world.

Figure 35 This slide was originally created by Dr. B. Krick/Passive House Institute Also for lighting, the improvement in efficiency was the most important parameter for achieving the currently customary level of comfort. It is still worthwhile to apply approaches in which light is used in a targeted way in the places where it is actually needed - and switch it off where it does not provide a benefit. However, with the level of efficiency available today, the provision of sufficient light at all times completely on the basis of renewable energy sources is possible, the couple of hours of electricity storage necessary for this is also easily available for such a small consumption.

Rebound effect?

In this example one can also discuss the question, what role the so called “rebound effect” can play. In order to “consume” the savings made possible by the improved light source, we would have to increase the luminous flux by a factor of 17. Of course, that is possible – but it’s actually not done in practice. Yes, there has been some improvement in illumination, it may even be by a factor 3. But that would still leave us energy savings of >83% instead of the possible 94%. Obviously, this does not matter very much – both savings are extremely high and both are high enough to be able to provide the energy needed solely by renewable generation. In order to “fight” rebound effects, the improvement in efficiency has just to be high enough, so that the remaining “demand” will be negligible anyhow. As is the case for passive houses, bicycle-technology based electric light weight vevicles and LED-light. For the specific case of passive houses that is discussed in more detail in [Johnston 2021]

This article will show how this is possible in many ways: Passive House buildings require such a small amount of heating energy that the remaining demand is no longer difficult to meet, lightweight electric vehicles based on bicycle technology can be supplied with power from solar panels on the roof of the house, and LED lights only have an extremely small electricity demand.

Moreover, there is no reason why we shouldn't think also about the meaning and purpose of further increased utilisation of energy services. Does the fact that today we can fly to holiday destinations in increasingly far-off regions three times a year instead of just once a year really improve the quality of our lives? Is a room temperature of 23°C in winter, implicitly agreed on for light clothing, really benefit us 57)? Do we really have to buy new lounge furniture every 8 years? Is our awareness of life really so much better at a speed of 180 than at 130? I know from experience with numerous discussions that these questions are capable of triggering huge emotional reactions, and on both sides by the way, each of which represents a resolute position. I think that it is especially important to reflect on this; in view of the urgent risks of the climate crisis and the time remaining to us for finding a solution, there are certainly important arguments in favour of reflecting on this.

The situation is different for efficiency potentials: these can in fact be implemented without having a debate on the virtues of abstinence; in many cases, there may even be an increase in the energy service, with the savings remaining high despite this. Everyone can benefit from exploiting these efficiency potentials, regardless of their position on these questions. ||

As an amateur astronomer, I would like to touch on a related subject: light pollution, which is increasing worldwide. We have become so used to the availability of cheap light all day long that in some places everything is lighted up even all throughout the night, with the consequence that often we can no longer see the stars in the sky, let alone the subtle band of the Milky Way - a real cultural deficit, robbing us of the connection to our cosmic home. Astronomic observations are increasingly becoming difficult.

With growing material prosperity, more and more people can now afford to consume more of previously scarce goods. In my view this undoubtedly constitutes progress in overcoming hunger, disease, homelessness and shameless exploitation - that must be kept in mind by those who seek a solution exclusively in abstinence. However, if I already have 500 lux available to me on my desk then the next 500 by far will no longer be such an enormous increase in the quality of life. If I demand that such amounts of light at least should not be emitted into the night sky through the windows is a legitimate demand for the quality of life of my fellow human beings, particularly that of astronomers.

Conclusion: even though a good level of lighting is very cheap, attention should still be given to less light leakage into the surroundings.

Milky Way observed from northern latitudes. This is our cosmic home - each of these bands consists of several hundreds of millions of stars. Picture: Fresh.waffles CC BY-SA 4.0.

Energy service: communication, information technology

Monitors account for a significant share of the energy consumption of information technology. While the CRT monitors (typical power con-sumption: 76 watts) that were mostly used a decade ago used to heat up, emitted x-rays, were heavy and required a lot of space, LCD monitors today already have a considerably better efficiency and are space-saving: 10-20 watts are typical for good devices. Now is that “good enough” or does this constitute the limit of physical possibilities? By no means: conventional printed books demon¬strate that no energy is actually necessary for displaying information (except for light, which only requires a few watts). Indeed, the next slide shows that the technology with very high efficiency does in fact already exist: reflective displays with pigment particles that can be adjusted via electrical fields known as “electronic ink” is already being used in some end devices.

Figure 36

Except for the light and minimal field changes during a screen change, the electronic paper does not consume any energy for the display; the image even remains permanently if the device is switched off. This technology is still relatively expensive, but this may change fast.

Figure 37 comfortabler technology=more comfortable technology

How relevant is this in practice?

Can the potentials identified here actually be exploited in economic terms for the entire national economy? This question can now be answered quite easily because practical implementation of at least some of these potentials has now been underway for some decades, at least in some countries. As an example, I will pick out the development in Germany - not because this is particularly “exemplary” 49) , but because the associated data is well-documented and has already been evaluated in detail for many years. To better assess the results, it should be added that for more than a decade, exploiting the efficiency potentials played almost no role in the public discourse. As we shall see, the considerable successes achieved in spite of this, arise mainly due to the small improvement steps in the case of manufactured products that occur anyway in the course of renewal processes in natural replacement cycles 50) .

Between 1990 and 2016 the economic performance (measured using the gross domestic product GDP) of Germany increased by approximately 51% in total. Here we will assume an increase in the energy services to roughly the same extent 51) . With an unchanged energy intensity (particularly efficiency), it had been expected against this background that the primary energy consumption PE in Germany would increase to the same extent, that is from 14913 PJ/a up to around 22500 PJ/a. But in fact the PE actually decreased during this period, namely to 13383 PJ/a (that's 10% savings). Thus in Germany, “efficiency energy” already covers more than 40% of the demand and therefore already represents the largest single share of all energy sources 52) . The annual average improvement in efficiency with 2.3%/a is even significantly higher than growth in energy services (1.9%/a).

Figure 38

That's a big success of the efficiency movement, particularly when we take into consideration that public policy and the business community did not place any particular emphasis on these improvements. Elsewhere we have explained how this success could be significantly increased: with a little commitment, a 3.3% increase in efficiency per year would be achievable and this would allow us to achieve the climate objectives relatively quickly 53) .

How can the increase in the use of efficiency be achieved specifically? Basically it is surprisingly simple: for example, each year about 3% of all windows in the existing building stock are renewed. So far these new windows have usually been better than the old ones by a factor of 2 (not that bad!). Altogether this results in a reduction in window-heat-losses of approximately 1.5%/a. If we assume the same rate of renewal but use of state of the art windows (i.e. passive house windows), then the savings will increase to more than 82% in each particular case, and that's 2.6%/a in this segment 54) . The approach here is thus: the new or replacement investments that are coming up are undertaken - but with several times improved components rather than “average products of a mediocre standard” 55) . How we can make even more of these improved products available will be shown in the next slide.

This slide shows a procedure for initiating the necessary improvements in efficiency that has been successfully used for many decades. Every manufacturer of a product which leads to energy flows can contribute to this 56) . The manufacturer considers a proposal for an improved product and contacts an energy efficiency certifier 57) , who has the competence to test the improved component for the degree of efficiency it actually achieves 58). If the criteria are met, the efficiency certificate can be issued. If that is not the case, then an engineer at the PHI will be able to provide guidance relating to any weak points that still exist and information about how these can be improved further. After a finite number of iterations, an improvement by several times is often achieved. This is how thousands of components that are significantly better in terms of efficiency have already come to market 59).

Figure 39

Another more complementary approach consists of integrated planning and quality assurance e.g. for construction processes 60): advanced planning tools such as the Passive House Planning Package allow architects and designers to optimise such designs also in terms of energy efficiency right from the start. Experience has shown that if used at an early stage, this can reduce the subsequent consumption of the buildings by several times compared to the “business as usual” approach - and renewable electricity generation on the roof or façade can also be integrated at the same time. This slide shows a colourful variety of new builds and modernised buildings with a verified Passive House standard - i.e. energy consumption values which are lower by a good five times than with the 'usual course of action'. Because this relates to new builds or modernisations that are already pending, the effort for this usually isn't very high compared to the “already necessary” case 61) .

Figure 40

Here are more information and additional links for further study: a low cost possibility for heating buildings with renewable energy is offered by simple air conditioning units (also called split units). These are now more efficient, quieter and more cost-efficient - and besides cooling can also provide heating with reasonable COPs. In the report "Heating with an air conditioning split unit" (only in German) measured results from two years of testing with such a split-device have been documented and discussed. Information and practical tips for energy saving measures have been compiled by the “Efficiency NOW” campaign of the Passive House Institute. These can be found online at

Figure 41

Improved energy efficiency has proved to be the key to a sustainable solution for the problems associated with energy supply. A careful analysis of current energy use, particularly of those systems that are at the end of the utilisation chain, has led us to quantification of energy services. In most cases, these energy services can be provided with energy expenditure close to zero, by choosing a (possibly controlled) passive technical solution instead of an active one. Consumption values will typically decrease this way to between a quarter and a twentieth of today's normal values. The switch will not happen “by itself” of course - but the necessary investments can be shoul-dered easily if the improvement is carried out in the course of the usual renewal cycle. Practical experiences with this have been gained in the meantime which make a transition that is also economically attractive appear possible within a few decades. A completely renewable energy supply system without enormous infrastructure costs becomes possible with this approach. Relevant national economies are already on this path; because the time is now critical due to the climate crisis, greater commitment will pay off.

Figure 42


[Bossel 1980] Hartmut Bossel Energiewende (Energy Transition), 1980, paperback ISBN: 3100077059

[Johnston 2020] David Johnston, Mark Siddall, Oliver Ottinger, Soeren Peper und Wolfgang Feist: Are the energy savings of the passive house standard reliable? A review of the as-built thermal and space heating performance of passive house dwellings from 1990 to 2018; Energy Efficiency (2020) 13:1605–1631;

[Lovins 1977] Amory and Hunter Lovins: Soft Energy Paths, 1977, ISBN-10: 0060906537

[Meyer-Abich 1979], Klaus Michael Meyer-Abich: Energieeinsparung als neue Energiequelle - Wirtschaftspolitische Möglichkeiten und alternative Technologien (Saving energy as a new energy source - Economic policy opportunities and alternative technologies), 1979 ISBN: 3446127348

[Nørgård 1979] Jørgen Stig Nørgård: Improved efficiency in domestic electricity use, Technical University of Denmark | DTU · Department of Civil Engineering, 1979

[Rosenfeld 2001] Arthur H. Rosenfeld, T. M. Kaarsberg, J. J. Romm, Efficiency of Energy Use, in The Macmillan Encyclopedia of Energy, John Zumerchik, editor in chief, Macmillan Reference USA, 2001. See also and

[Shurcliff 1981] William Shurcliff: Super Insulated Houses and Double Envelope Houses, Brick House, Andover, 1st edition 1981

It took some time
We will discuss the central importance of the concept of energy in physics elsewhere: Most physicists would agree if we say here that in physics there are really only two basic building blocks: energy and information
Or it is simply not understood
Who learnt math in school knows that such a question depends on the reference. If my capability to deliver work is the reference, that 4120 Joule is quite a lot of energy. But you might have a different reference. The calculation also introduces the SI-unit of energy, namely the “J” (Joule). It's the energy of the work done lifting one kg against 1 m/s² up one metre: 1 J = 1 kg m²/s² = 1 Nm.
The kWh is a “derived legal unit” and therefore also permissible; and quite a practical unit
Isn't it too confusing if, after very careful analysis, we must now determine that in practice 'closed systems' always exist only approximately? Yes, we can keep 'crossing of boundaries' very small using sophisticated technology, but 'zero'? Not in an environment in which people may also be present. The “closed system” is thus an idealistic conception, and perhaps it is even reassuring: contrary to what many believe, even a Passive House building is an open system. Only, the flows of heat losses are so small that it doesn't bother anyone, and we can work in sufficiently good approximation with the “closed system model” for energy balances, even though e.g. considerable classic material flows such as fresh air and tap water are constantly passing through the systems boundary
due to the use of energy for the open system
by analogy with a mass balance
this is how a machine can be used in continuous operation in an economically optimal way
energy recovery
or the same
dark red vertical line with dashes
this is true for a majority of all energy flows
this sometimes happens: e.g. if the piano stays at the raised level on the second floor or if a produced raw material (e.g. aluminum) is used in a stable way as a product
which is the case in approximately 95% of all applications, as we will see later on
The system itself is called the service system and the energy which is fed into this as an input is referred to as “useful energy”“.
not to be recovered in 99% of all cases (e.g.: we do not recover heat from the breaks in our cars
without heat supply to the coffee pot in Figure 2 the coffee will become really cold and there will be no energy service
temperature °C on an living area during the time period
thus the loss of heat flow
but we normally don't undertake any effort for this; that would be very expensive
This can be calculated in every specific case using thePHPP software.
That's like a weighing scale
based on the living area
the relevant wind turbine can also be situated somewhere else; in our case it was just a couple of km away from the building
namely triple low-e glazing
Not 100%; neither is this necessary since sustainable energy source ARE available.
in a zero-heating-energy house; this is possible in most climates – but we do not recommend that, see
like we did for the coffee machine earlier on
Although the technical efficiency of the motors has increased considerably in these periods. In this area the industry has applied improved efficiency almost exclusively for further increasing the engine power, vehicle weight and the final speed. This is often called the “rebound effect”. In the present case the reason is different: The main goal of the industry had been to promote cars that are more powerful, faster and heavier. We do not wish to discuss the reasonableness of such objectives here, but we do concede that this debate is necessary. But: without the improved efficiency the energy consumption would be even much higher than it is right now; this was actually planned for: All the energy prognosis of the eighties had growing ‘demand’ as a result and the fossil fuel industry did not enjoy, that in Europe (and also in America) that growth has been reduced and even reversed.
Of course, some attention must be paid to how the electricity for this is produced ultimately - but even fossil fuel based modern gas-fired power plants have efficiencies higher than 50%, and even with such electricity the overall system is still always far better than using an internal combustion engine.
Due to integration into the entire industry system, the situation altogether is still a little bit “convoluted”. Thus it is really important to know where the focus on power generation comes from; however, it is foreseeable that for electricity we can switch to largely renewable energy generation worldwide. Electric propulsion systems will then become even better in the overall assessment, and due to the high efficiency of electric motors the overall electricity demand for transportation will then no longer be so exorbitantly high, so that this will be quite manageable for a renewable energy system. Also, there is not much seasonal change in energy demand for transportation; at least not as much as with heating. This also make the transformation to electric vehicles less complicated. Using batteries in the vehicles will definitely be the most attractive solution in >90% of all cases.
The “faster, heavier and bigger” fraction has prevailed, going as far as the consequences that were revealed by the scandal uncovered a short time later.
'Crazy how much work it is to toast it'
Note about the above-mentioned 2.4 kWh/(100 km): The value of 0.9 is solely the mechanical energy which a human can 'generate' with his muscles with an efficiency of ca. 30%. The “bread roll input” in our case is 0.9 kWh/30% = 3 kWh, only a little higher than the average value in Slide 20. The normal cyclist “only” travels at a comfortable 20 km/h. By the way: approximately 0.6 kWh must actually still be deducted for the human basic metabolic rate (output necessary in any case) in both cases.
Because it is streamlined and has brake energy recovery.
except for the first and last journey, but both these account for only a negligible share of the travelled distances
precisely the integral
at least in very good approximation
Incidentally, energy is converted in the process: in winter potential energy of the gravitational field of the sun is converted into more kinetic energy of the planet in the orbit (but total energy is still conserved, of course), the Earth becomes slower again after the solstice, using up kinetic energy to reach the point furthest from the sun again in the summer; this has already been going on for more than 4 billion years without any significant energy being 'consumed'. Remark: according to the general theory of relativity, gravitational energy is in fact emitted to an extremely small extent in the process.
The main “reason” why we have achieved less progress in this regard than is actually possible, is the mindset: society takes pride in incurring increasingly greater outputs and greater expenditure for its purposes, even if these increases ultimately hardly bring anything and are often beginning to be destructive. An association with the idea of sufficiency exists here - in an extreme sense, because avoidable inefficiency by no means improves the quality of life. It then only serves to maintain a destructive ideology.
This is the only boundary condition, so there is no propulsion and no friction; however, with “very little friction” there is hardly any difference in the outcome.
Unless it goes down much too steep, so that the body will have to consume part of the horizontal velocity.
at least to a certain degree
This is indeed very similar tot he process of borrowing a tool for a specific task; the tool will (almost) not change and you can return it after the task is done
total sum of all sectors
In contrast with most other energy services, the final energy demand for artificial lighting can only be reduced to a physical minimum of >0, but we are still far from reaching this - and the total energy needed for this is very little anyway.
this is fairly typical for developed industrialized nations in the past decades
E.g. replacing windows or lamps or outdated monitors…
Indeed, developed living area has increased and there are more cars. In principle, it would be possible to calculate the total Energy-Service; strictly speaking this only requires diligence, because e.g. the number of vehicle kilometers have been recorded and published. A rough estimate supports the hypothesis that energy services have in fact more or less increased with the GDP.
If we follow the approach of our colleague Meyer-Abich, who has spoken of “saving energy as an energy source” with regard to the possibilities for substitution of energy sources through “saved energy”. This (early) publication also shows that the insights presented here are by no means 'new'; [Lovins], [Bossel], [Nørgård], [Rosenfeld] and [Shurcliff] also correctly explained these contexts in the 1970s.
together with a roughly equal expansion of renewable energy generation; both complement each other perfectly
in view of the fossil fuel supply crisis and the high economic efficiency, a higher rate should certainly be aimed for
all of which by the way are already available on the market at reasonable cost
Illustration using an example of a window frame. The same goes for e.g. an AC adapter or a car roof rack or an elevator or
in this case this relates to the “Passive House Approved” certificate
for a window, this mainly relates to the heat losses which are often considerably higher over the usage duration than e.g. the manufacturing energy for such a frame
By the way, for governments it would be relatively easy to intensify the push for these processes by offering incentives for successful improvements; in most cases, the costs for such developments are not particularly high.
New build or retrofit. Such an approach is also expedient for other complex systems such as the new development of a car
Example: applying thermal insulation instead of just new plasterwork
efficiency_now/the_big_picture.txt · Last modified: 2023/01/24 18:24 by wfeist