planning:thermal_protection:thermal_protection_works:thermal_protection_vs._thermal_storage
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planning:thermal_protection:thermal_protection_works:thermal_protection_vs._thermal_storage [2012/04/20 13:02] – sarah | planning:thermal_protection:thermal_protection_works:thermal_protection_vs._thermal_storage [2022/02/15 19:57] (current) – admin | ||
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+ | ====== Insulation vs. thermal mass ====== | ||
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+ | In some publications, | ||
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+ | The author of this article has already dealt with this topic systematically in 1987, under the same title. In the meantime, many new findings have become available, all of which support this publication. The full (German) version of the summary given here can be ordered from the following link: [[https:// | ||
+ | \\ | ||
+ | ===== The main facts ===== | ||
+ | |||
+ | The latest research proves ((**Some accuse the scientific community of applying laws (e.g. heat conductivity or the second law of thermodynamics) that have not been " | ||
+ | |||
+ | Furthermore, | ||
+ | * What happens, when the heating system of an old building breaks down in winter? The author himself experienced that: the temperatures can sink to below zero – the water in the flower vase froze. | ||
+ | |||
+ | * And what happens when the heating in a Passive House breaks down? Even at minus temperatures, | ||
+ | \\ | ||
+ | Simply stated: although it is not " | ||
+ | \\ | ||
+ | On the face of it, reputable science "has a harder time" than fanatics who believe in the absoluteness of their convictions.\\ | ||
+ | \\ | ||
+ | Science has always stood up to examination time and time again, but there is a double advantage in that. On the one hand, this guarantees a process of continued improvement.\\ | ||
+ | \\ | ||
+ | And on the other hand, this teaches tolerance. No one possesses absolute truth. One's own conviction can never be important enough to call into question the dignity of other people. Ethical principles are above science. Oh, if only this was finally generally accepted! (see Max Born: My Life and My Views: A Nobel Prize Winner in Physics Writes Provocatively on a Wide Range of Subjects (translation of "Von der Verantwortung des Naturwissenschaftlers”) ).\\ | ||
+ | \\ | ||
+ | Incidentally: | ||
+ | * The thermal protection of the external envelope (U-value) and the air exchange are mainly responsible for the heating energy consumption of a house in Central Europe. | ||
+ | |||
+ | * Irradiation on external wall surfaces in the average heating period is usually an insignificant effect with very small energy gains, which is reduced even more by the heat radiation into the cold sky. However, the passive use of solar energy can be considerably increased through measures such as a selective coating or a transparent (translucent) insulation. | ||
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+ | * After all, the influence of the thermal storage capability of the external walls is extremely small (less than 0.5 %). | ||
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+ | * The heat capacity of the interior building components facing towards the interior has a perceptible influence on the temperature stability and thus on summer comfort – the interior walls and intermediate ceilings are important.\\ | ||
+ | \\ | ||
+ | Evidence of these facts is provided and explained in detail in the full version. The main conclusions are asfollows: | ||
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+ | * For external building components, it is the insulation that is effective against heat losses. Whether internal or external - insulation is always efficient. However, the prevention of constructive thermal bridges and airtightness are essential for the effective functioning of the insulation. | ||
+ | |||
+ | * The thermal storage capacity of the external building components is insignificant. | ||
+ | |||
+ | * The absorptivity of the external surface for solar energy and the emissivity of the surfaces for long-wave heat emission is important only to a small extent.\\ | ||
+ | \\ | ||
+ | ===== Definition of thermal storage ===== | ||
+ | |||
+ | The thermal storage capacity or specific heat capacity (this is the term used in physics) is defined as **the ability of a material to take up heat quantities in a temperature gradient. **We have been using this storage effect for a long time, e.g. for hot water bottles, boilers or storage heaters. Basically, heat storage does not provide additional energy – every quantity of heat taken from storage must originally have been supplied to the storage, e.g. byheating the water for a hot water bottle.\\ | ||
+ | \\ | ||
+ | ===== Self-discharge ===== | ||
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+ | An uninsulated hot water bottle (one which is not under a well-insulating duvet) releases its heat content within a short time and becomes a "cold water bottle" | ||
+ | \\ | ||
+ | |{{ : | ||
+ | \\ | ||
+ | ===== Thermal protection and thermal storage complement each other ===== | ||
+ | |||
+ | Both are described by the basic equation for heat transport. This has been known in physics since 1822, when [[http:// | ||
+ | |||
+ | |||
+ | <WRAP center 60%> | ||
+ | $$\rho c \dfrac{\delta T}{\delta t} = - div\,(- \Lambda\, | ||
+ | </ | ||
+ | The heat equation in general formulation describes the time variation of a temperature field T(x,y,z) in fixed matter (e.g. in a solid body). | ||
+ | |||
+ | * Differences in the temperature (gradient //grad//, on the right) propel a heat flux which increases proportional to the relevant component of the thermal conductivity tensor $\Lambda$. ((The most general formulation with which the thermal conductivity can vary for different spatial directions (e.g. in a perforated brick) is represented here. If the thermal conductivity is invariant with respect to direction (isotropic), | ||
+ | |||
+ | * The negative divergence of the heat flow is the change of the heat content in the infinitesimal volume element. | ||
+ | |||
+ | * This is the same as the temporal change in temperature $\frac{\partial T}{\partial t}$ multiplied by the heat capacity $\rho c$(left side of equation). | ||
+ | This equation has proved to be consistently effective in physics and technology. Such different things like heat transfer in stars, in semi-conductor devices, brake pads and many others can be calculated in good correlation with measurements. This equation also applies in building physics – and the calculations made using it correspond just as well with building physical measurements as shown in [[planning: | ||
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+ | Today it is possible to apply this differential equation with the help of mathematical software for various wall structures for example, and thereby obtain an exact representation of the temperature courses varying with time. Programmes like HEAT2 or HEAT3 can even do this for two or three dimensions. The values thus calculated correspond very well with measurements. The same is true for processes varying with time. | ||
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+ | Also simulation programmes (e.g. " | ||
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+ | * For normal building components, it turns out that to a great extent, the heat storage effect already averages out over a period of a few days (see the explanation in [[planning: | ||
+ | |||
+ | * " | ||
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+ | * In simulations of complete buildings using Fourier' | ||
+ | \\ | ||
+ | |||
+ | ===== Stationary approximation ===== | ||
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+ | If long periods of time are observed, the inflow and outflow of energy for the heat capacity can be averaged out from the energy balance, because the same amount of energy has to be stored up as the amount which is available at the end again, if the temperatures at the beginning and end are the same.\\ | ||
+ | \\ | ||
+ | -> How long are "long periods of time"? This depends on the system being considered. | ||
+ | * For a sheet of paper, one hour is " | ||
+ | |||
+ | * for a 160 mm thick concrete ceiling three days are " | ||
+ | |||
+ | * however, for a several meter thick layer of earth, 6 years would be " | ||
+ | \\ | ||
+ | Such solid buildings are unsuitable for significant storage " | ||
+ | \\ | ||
+ | Stationary approximation can be used successfully for ordinary building components in building envelopes when heat losses during the heating period are being considered, because then the temperatures at the beginning and the end are about the same and the net storage balance is zero. This approximation leads to the well-known thermal transmittance coefficient or U-value (formerly k-value). Calculations using the U-value are sufficiently accurate for buildings of various types; for example, the simplified method of the Passive House Planning Package (PHPP) uses this approximation – and the results are in good agreement with measured results (see the page about [[Planning: | ||
+ | \\ | ||
+ | |||
+ | |||
+ | ===== Theory and practice (measurement) ===== | ||
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+ | How well theory and practice correlate regarding heat conductance is shown by the temperature curves of the measurements recorded during the monitoring programme for the Passive House in Kranichstein. The two graphs show the measured values (coloured symbols). The results of the calculation with the simulation model are represented by black lines. The correlation between the measurement and the theory is so good that differences can only be identified by magnification of the image (magnifying glass). Any deviations present are +/- 0.2 °C at the most.\\ | ||
+ | \\ | ||
+ | |{{ : | ||
+ | |// | ||
+ | - remaining errors in calibration of sensors (about +- 0.15 K)\\ | ||
+ | - remaining inaccuracy of the position of sensors (about +- 2 mm)\\ | ||
+ | - disturbance in the wall build-up due to e.g. varying mortar thicknesses\\ | ||
+ | - influence of thermal bridges present further away (the calculation is only made using the one-dimensional heat conductance equation)\\ | ||
+ | - effects of moisture resorption (which in principle could have been taken into account in coupled heat and moisture transport equations)\\ | ||
+ | - limited accuracy with regard to knowledge of material properties \\ | ||
+ | \\ | ||
+ | All these influences were checked as carefully as possible; this results in the comparatively good agreement of the measured values with the calculations. High experimental accuracy is an important prerequisite for actually measuring the parameters that are intended.))**// | ||
+ | \\ | ||
+ | The wall build-up and the position of the highly accurate measuring points of the Pt100 sensors is documented in this note ((**The measurements were carried out using Pt100 sensors that had been calibrated in the laboratory and were built into exactly measured points inside the wall; measurements were recorded over a period of many years.** The measuring lines were laid so that they did not affect the temperature field and the heat emitted by the sensors was negligible (four-conductor measurement with brief electrical impulses only during the measurement). The monitoring programme was funded by the Hessian State government. Results were published in the Protocol Volume No.5 “Energy balance and temperature Characteristics” of the Research Group for Cost-efficient Passive Houses, among others. More detailed information about the mathematical calculations can also be found in it [[planning: | ||
+ | - remaining errors in calibration of sensors (about +- 0.15 K)\\ | ||
+ | - remaining inaccuracy of the position of sensors (about +- 2 mm)\\ | ||
+ | - disturbance in the wall build-up due to e.g. varying mortar thicknesses\\ | ||
+ | - influence of thermal bridges present further away (the calculation is only made using the one-dimensional heat conductance equation)\\ | ||
+ | - effects of moisture resorption (which in principle could have been taken into account in coupled heat and moisture transport equations)\\ | ||
+ | - limited accuracy with regard to knowledge of material properties \\ | ||
+ | \\ | ||
+ | All these influences were checked as carefully as possible; this results in the comparatively good agreement of the measured values with the calculations. High experimental accuracy is an important prerequisite for actually measuring the parameters that are intended.)). The insulation layer was 275 mm thick. Based on these results, many other characteristics of the insulated wall become apparent – a more detailed discussion of these can be found in [[planning: | ||
+ | \\ | ||
+ | ===== In contrast: the total interior heat capacity does have an influence ===== | ||
+ | |||
+ | What is meant by the (effective) interior heat capacity? That is the total heat capacity associated with the room through the interior surfaces of all the building elements on the inside. It is inside the insulating envelope, like the fluid inside an insulated thermos flask. This heat capacity has a cushioning effect on temperature changes in the room, e.g. those due to solar radiation through the windows. In the main heating period this is not really important - but in the summer, when mainly the daily peaks in temperature have to be reduced and night-time cooling is possible, the interior heat capacity is advantageous. Good insulation is also helpful in the summer because it reduces the infiltration of heat into the rooms.\\ | ||
+ | \\ | ||
+ | ===== Conclusion and examples ===== | ||
+ | |||
+ | It is the insulation that matters and not the heat capacity. This is true **not only for buildings**, | ||
+ | |||
+ | * If we want to keep tea or coffee hot, we use a tea-cosy or thermos flask – the alternative to insulation is not storage but constant energy expenditure for heating (tea-light or hot-plate). | ||
+ | |||
+ | * In cold weather we put on insulating jumpers, stockings, hats etc. | ||
+ | |||
+ | * In cold bedrooms, we keep beds warm by using “warm” duvets. Of course, the duvet itself is not warm, it is just very insulating, so that the human body loses less heat. | ||
+ | |||
+ | * Farmers are warned regularly about the occurrence of ground frost. Frost always occurs on the ground first because of the heat emitted into the night sky (in spite of thermal storage and solar radiation). The farmer can protect his plants with hay (insulation!) or sheeting (translucent insulation).\\ | ||
+ | \\ | ||
+ | The best evidence for the effectiveness of good insulation is the **Passive House** itself. In autumn the Passive House remains warm for a long time because it **loses very little heat due to the excellent insulation and heat recovery**. Even if heating is needed in winter, | ||
+ | \\ | ||
+ | The scientific context can be checked by anyone – no authority by any Guru is required for this. Incidentally, | ||
+ | \\ | ||
+ | ==== Learning through work and action ==== | ||
+ | |||
+ | The topic discussed here is very suitable for school projects. In secondary education and sixth form classes a fundamental understanding of physics and the differences between extensive properties (like enthalpy/ | ||
+ | \\ | ||
+ | |||
+ | ===== See also ===== | ||
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+ | [[planning: | ||
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+ | [[planning: | ||
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+ | ===== Literature ===== | ||
+ | |||
+ | **[Feist 1987]** Ist Wärmespeichern wichtiger als Wärmedämmen? | ||
+ | (**“Is thermal storage more important than thermal insulation? | ||
+ | \\ | ||
+ | **[Feist 1993]** Passivhäuser in Mitteleuropa; | ||
+ | (**“Passive houses in Central Europe”**; | ||
+ | \\ | ||
+ | **[AkkP 5]** Energiebilanz und Temperaturverhalten; | ||
+ | (**“Energy balance and temperature behaviour”**; | ||