While renewable energy sources accounted for 45.4 % of Germany’s gross electricity consumption in 2020, they are making much slower entry into heating and cooling supply (1). In the same year, for example, the share of renewable energy sources in final energy consumption for heating and cooling was only 15.2 % (1), although the heating market alone, such as the provision of space heating, hot water and other process heat, has a share of 54.1 % of total final energy consumption in Germany (as of 2017; 2).
Globally speaking, the rapidly increasing demand for space cooling probably represents one of the greatest energy technology challenges of our time. Today, the use of air conditioning systems and fans already accounts for around one fifth of global electricity consumption in buildings or one tenth of total global electricity consumption. According to an estimate by the International Energy Agency, current demand could even more than triple by 2050 (3).
Not least against the background of the immense challenges mentioned above, there are repeated calls for the accelerated use of efficient heat pumps and chillers. Taking up this discussion, the following section provides an outline of the basic principles and possible forms of application as well as a brief assessment of the environmental compatibility and future viability of these technologies.
As a general principle, heat moves along a temperature gradient, such as from warmer to cooler substances. Heat pumps and chillers circumvent this law of nature, which makes it possible to move heat from cooler to warmer areas. When heat is extracted from the outside and transported to the inside of a building, it is generally referred to as a heat pump; its use is for heating. On the other hand, when heat is extracted from an interior space and transferred to the outside, it is generally referred to as a chiller; its benefit is cooling.
Compression heat pump/chiller
This technology is usually driven by an electric motor, but internal combustion engines are also used in some cases. The basic principle is shown in Figure 1 using the example of a heat pump.
Figure 1: Principle of the absorption heat pump (own, qualitative representation according to (4))
A refrigerant with a low evaporation temperature circulates in the heat pump and is evaporated in a heat exchanger, the evaporator, by absorbing ambient heat. The gaseous refrigerant is then being compressed, which greatly increases both its pressure and its temperature. The compressor must be driven externally with energy. The refrigerant is then liquefied again in a further heat exchanger, the condenser, whereby the thermal energy released in this process represents the actual benefit, such as space heating or water heating. Finally, the refrigerant is returned to the evaporator via an expansion valve, reducing its pressure and temperature. The entire process is referred to as a left-hand cycle, as a counterpart to the right-hand cycle used in a thermal power plant.
In contrast to the heat pump, the benefit of the refrigerating machine, such as refrigerators, is the absorbed heat energy by the evaporator, which is extracted from the place to be cooled. Apart from the associated deviating temperature levels for heat input and output, the mode of operation of the refrigerating machine is basically identical to the above-mentioned mode of operation of the heat pump.
Absorption heat pump/cooling machine
In this technology, the mechanical compressor is replaced by a thermal compressor. Its basic design is shown in Figure 2, using the heat pump as an example.
Figure 2: Principle of the absorption heat pump (based on (4))
After absorbing ambient heat, the refrigerant vapor is fed into an absorber where it is dissolved in an absorbent, releasing usable heat energy. This mixture, which is rich in refrigerant, is transported to the discharger with the aid of a solvent pump. There, heat energy is supplied and the refrigerant is evaporated from the solution again. The remaining refrigerant-depleted solution is returned to the absorber via an expansion valve, while the refrigerant vapor, releasing useful heat, follows the path previously illustrated for the compression heat pump.
The advantage of this technology is that pressure increase of liquid refrigerants require only a fraction of the mechanical energy which is needed to compress the refrigerant vapor in the compression heat pump. In the absorption heat pump, the actual external drive is the thermal energy supplied to the expander, which can, for example, be provided by solar or geothermal energy or by burning natural gas or biogas.
Analogous to the compression technology, absorption heat pumps can also be operated as chillers. In this context, the possibility of solar thermal cooling resulting from this technology should be mentioned, which will be considered in more detail in this article.
Heat sources and efficiencies
Even in the depths of winter heat pumps make use of available ambient heat and harness it by supplying external energy. Their efficiency increases with rising temperature levels of the heat source, the latter being primarily outside air, groundwater, or the ground.
The use of outside air has the advantage that it is available everywhere and can be used easily. The disadvantage, however, is that the potential heat gain varies greatly due to seasonal and diurnal fluctuations in air temperature, and behaves in the opposite direction to the heating requirement. The temperature in the ground on the other hand is from a certain depth mainly influenced by the natural heat from the earth’s interior, which results in significantly lower seasonal fluctuations. Groundwater also has the advantage of a comparatively constant high temperature level.
Furthermore, the lower the temperature required on the heat output side, the more efficiently the heat pump works. For this reason, underfloor heating, for example, with its lower flow temperature, is preferable to a conventional radiator in this context.
The useful heat output of the heat pump is made up of the thermal energy absorbed from the heat source and externally supplied drive energy. The ratio of the useful heat output and the externally supplied drive energy determines the efficiency, which is represented in the compression heat pump in the form of the so-called coefficient of performance. Due to seasonally fluctuating temperature conditions, the coefficient of performance varies, which is why the so-called annual performance factor is used as an average of the coefficients of performance to evaluate compression heat pumps. Modern heat pumps of this type can achieve an annual performance factor above 5 when using the ground as a heat source under optimal conditions (including new building construction with underfloor heating), which means that on average they supply more than five kilowatt hours of heat energy per kilowatt hour of electrical energy consumed per year (5). However, when using outside air and under very poor conditions, the annual performance factor can also drop to values of about 1.5 (5), although a value of about 3 can be assumed as a reasonable lower limit given the current German electricity mix (6). Table 1 presents the average annual performance factors of electrically driven compression heat pumps as a function of various parameters.
Table 1: Average annual performance factors of compression heat pumps (5)
2.6 to 3.3
2.4 to 2.7
3.2 to 4.3
2.9 to 3.3
Comparable to soil
Comparable to soil
Regarding the above values, it should be noted that groundwater heat pumps require a relatively large amount of energy to operate the groundwater pump, which is why their annual performance factor is somewhat lower than the well-suited heat source would generally lead one to expect (5).
A coefficient of performance can also be specified for chillers. In this case, it results from the ratio of extracted heat energy (cooling capacity) and externally supplied drive energy. In the case of compression refrigeration machines, it is sometimes also referred to as the refrigeration coefficient of performance. At the same temperature levels, it is lower than for the compression heat pump because the benefit in terms of the cold provided (extracted heat energy) is less by the drive energy than the delivered heat energy, which is the benefit of the heat pump. Nevertheless, chillers often achieve higher coefficients of performance than heat pumps because they usually operate with lower temperature differences (7).
For absorption chillers, the coefficient of performance is referred to as the heat ratio, which relates the cold provided to the heat energy supplied. For example, a machine with a heat ratio of 2 provides twice as much cold as it consumes in heat energy to drive it. In some circumstances, the heat ratio may be well below 1, but the use of an absorption chiller may be appropriate even in such a case, provided that otherwise unusable waste heat or solar heat is used for the drive (8).
Solar cooling use case
The increasing global demand for space cooling is a challenge in the energy sector which is currently still often ignored. Yet life without cooling devices, e. g. refrigerators and air conditioners, is almost inconceivable. Although around 90 % of all people in industrialized countries such as the USA and Japan already have air conditioning, only around 8 % of the 2.8 billion inhabitants of the hottest regions on earth do (3). According to the ‘Sustainable Energy for All’ organization, this lack of cooling systems threatens one billion people, especially in Asia and Africa (9). The WHO specifically estimates that nearly half a million people die each year from eating spoiled food – spoiled usually because of a lack of refrigeration facilities (9). Furthermore, interruptions in the cold chain for vaccines cause major problems, not least in the era of COVID-19. According to the International Energy Agency, the rising affluence of countries in the global South will drive electricity consumption for refrigeration in the future, with the emerging economies of China, India, and Indonesia predicted to be major players in this regard by 2050 (3).
Among all the possible applications and operations of the technologies described in this article, provision of cooling by using solar or thermal energy is a particularly interesting application in light of the above-mentioned issues. The great advantage here is that the cooling demand and the availability of solar drive energy tend to coincide in time and space.
Both the combination of a photovoltaic system with a compression refrigeration system (solar-electric cooling) and the combination of an absorption or adsorption chiller with solar thermal collectors (solar-thermal cooling) are possible. In contrast to the absorption chillers described above, adsorption chillers work with a solid sorbent. In general, both solar-electric and solar-thermal cooling require additional storage to bridge interruptions in solar irradiation. In principle, batteries or heat storage can be used to temporarily drive the chiller, and chilled water and ice storage can be used to temporarily cover the cooling demand directly.
An example of the practical implementation of the above technologies is the solar thermal cooling system shown in Figure 3, which is installed in a building of the CABR (China Academy of Building Research) in Beijing (10). It consists of solar thermal collectors with an overall area of 524 m² and an absorption chiller with a capacity of 176 kW (10). A hot and cold water storage tank with a volume of 15 and 8 m³, respectively, and a biomass boiler (232 kW) complete the system (10).
Figure 3: Solar thermal cooling system in Beijing, China (10)
The basic prerequisite for an ecologically clearly advantageous, CO2-neutral heating and cooling supply using the technologies described in this article is the use of renewable driving source of energy. In the case of operation with conventional energy sources, the ecological advantages may dwindle to a minimum. For example, the use of coal-fired electricity almost makes up for the efficiency advantage of a compression heat pump due to its high primary energy input, so that in this case the use of a modern natural gas heating system is hardly worse from a climate point of view (4).
Furthermore, the choice of refrigerant has a major influence on environmental compatibility. For example, compression heat pumps and chillers today largely use hydrofluorocarbons (HFCs) or hydrofluorocarbons (HFCs) as refrigerants. Although these have no negative impact on the ozone layer compared to the CFCs often used in the past, they do make an increased contribution to the greenhouse effect when released, e. g. through leakage. Natural refrigerants such as propane or CO2 are the better choice at this point, although the use of propane requires special safety precautions due to its high flammability. Ammonia, which is also a natural refrigerant, has the disadvantage of being toxic.
In summary, it can thus be said that although heat pumps and chillers have enormous ecological potential, this can only be fully exploited when renewable energies and non-critical refrigerants are used.
Particularly compression chillers have been in widespread use for decades as they are frequently used, in household refrigerators, freezers and air conditioners, for example. Heat pumps, on the other hand, experienced their first heyday at the time of the oil crisis, but the market collapsed again as oil prices fell (4). For some years now, however, the construction of heat pumps has been increasing noticeably again, most recently in 2020 with a growth rate of 40 % compared to the previous year (11).
Meanwhile, the heat pump is often referred to as a key technology in the heat transition, as it enables heat generation to be linked with electricity generation based on renewable energies. Potentially, by converting electricity surpluses from solar and wind energy, it is possible to avoid to a large extent both the regulation of photovoltaic and wind energy plants and the comparatively expensive energy storage in electricity storage facilities. The heat generated in this way can then either be used directly or stored comparatively cheaply in thermal energy storage systems. In addition, the use of electricity in the heating market has the general advantage that it is one of the few ways to generate heat without environmental pollution at the point of origin. Although this is also possible using heating rods (efficiency: almost 100%), heat pumps tend to be many times more energy efficient due to the additional use of freely available ambient heat.
According to a study by the Agora Energiewende think tank from 2017, around two million heat pumps could be installed in Germany by 2030 (12). However, according to the scenario developed in the study, five to six million heat pumps would be required in 2030 to achieve the oil phase-out in the heating sector and meet the climate targets of the German government that were still valid at the time the study was prepared (12). This is a demand that is unlikely to have become smaller as a result of the now more stringent German climate protection targets – now 65 % instead of 55 % greenhouse gas savings by 2030 compared with 1990 (13).
BY: TIM WEHRENBERG
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