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.

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.

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.

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.

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.

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.

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.

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.

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.

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$.

“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).

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.

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 $E_{out}$ :

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

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⁾.

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⁾).

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.

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.

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.

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.

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⁾.

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⁾.

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.

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.

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.

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

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.

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 (= s_ES) (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.

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.

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⁾.

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⁾.

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.

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.

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.

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.

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.

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.

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.

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).

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⁾.

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⁾ .

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 http://www.passipedia.de.

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.