Air conditioning in buildings
Active cooling of interiors using cold-generating systems becomes necessary in particular when the internal and external cooling loads of a building can no longer be dissipated effectively via night-time cooling or sustainable technologies such as earth sensors. The cold required to meet this demand can be produced using conventional compressor technology, simple adiabatic cooling or solar cooling technology. In Central Europe, for example, it can be assumed that there will be a demand in the range of 50 to 200 cooling hours per year, if thermal comfort indoors is to be maintained in residential buildings on all days of the year. In administration buildings the demand is at around 1,000 cooling hours per year.
Currently, almost exclusively compression-based cooling systems are used for air conditioning buildings. The Japan Refrigeration and Air Conditioning Industry Association (JRAIA), whose membership comprises the majority of Japanese manufacturers in this area, estimates that almost 69 million small compression cooling devices were sold worldwide in the past year alone. Solar cooling systems, on the other hand, hardly feature at all. In Europe currently only around 120 such systems are installed with an overall cooling output of around 12 MW.
In conventional compression technology, cold is produced by the electrically driven compression and subsequent decompression of a refrigerant. When in operation, this type of system has a high energy consumption, particularly at low outputs. Solar cooling technologies produce cold from heat. Compression systems have a higher COP (coefficient of performance; ratio of useful energy acquired and energy applied: COP=Eu /Ea) than solar cooling systems, but because of the different ways in which the two systems are driven, no direct comparison can be made.
Solar air conditioning
The power to operate solar cooling can be drawn from the heat harvested by solar collectors or the waste heat from a coupling process (e.g. of a co-generation plant). The use of solar energy is recommended when a significant proportion of the cooling load derives from external (solar) loads. In these cases the times when the cooling demand is at its highest coincides almost exactly with the time of maximum output of the system, which in turn is determined by the level of solar radiation. In Germany a solar radiation value of 940–1220 kWh/m²a is taken as the average. The electricity consumed by solar cooling systems is restricted to that needed to operate fans and pumps, so, depending on how the electricity was produced, they contribute only minimally to the production of anthropogenic CO2. It is recognised that the production of green electricity has relatively little global warming potential. One other benefit is that the use of solar cooling systems helps relieve peak loads within the electricity network, which in turn helps to avoid any overloading of the grid.
In general there are two types of solar air-conditioning system: open and closed. The closed systems include adsorption and absorption chillers, which produce cold water. DEC systems (desiccant evaporative cooling), also referred to as sorption-supported air conditioners, are open systems which generate cold air. In contrast to compression cooling systems, the sorption cooling systems and DEC systems do not involve mechanical compression of the refrigerant. Rather the refrigerant is taken up (sorption) by a hygroscopic solid or liquid and then driven out of the sorbent again (desorption) through the application of heat. To maintain the alternating process of sorption and desorption it is necessary to continuously apply heat, for example in the form of solar-thermal energy.
Absorption chillers operate on a liquid substance (absorbent), e.g. a lithium bromide solution. Adsorption chillers work with a solid (adsorbent), e.g. silica gel or zeolite. In addition to the systems available on the market, there are also steam jet cooling systems which are currently undergoing testing. Systems of this type use steam to compress the working substance.
Absorption and adsorption cooling systems
Sorption cooling systems operate with an extreme low pressure of around 10 Pa at which the refrigerant water can evaporate at a relatively low temperature. To keep the process going, the water vapour arising as a result of evaporating the refrigerant has to be constantly removed, and this happens via sorption.
Worldwide single-stage absorption chillers dominate the market. Unlike the two-stage systems, they can be operated with lower input temperatures, but are less efficient. Their drive temperature is around 75 to 100 °C, the coefficient of performance (COP) around 0.7. Also significant, in addition to the effectiveness of the system, is the temperature of cold water that is required, depending on the requirements of the building technology. Temperatures of roughly 6 to 20 °C can be achieved with a single-stage system; two-stage systems have a higher COP, at around 1.2, but they need higher drive temperatures of around 140 °C to 170 °C. A high level of waste heat is needed to run them, or high-performance collector systems have to be used, such as single-axis parabolic trough collectors with a tracking system. In Central Europe, however, such collectors are simply not an option, because there is not enough direct solar radiation in these latitudes. The cold-water temperatures achievable with a two-stage system are comparable with those of a single-stage system.
The cooling capacity achievable with absorption cooling systems is dependent on the size of the system installed. Currently available on the market are single-stage absorption cooling systems with a cooling capacity starting at 15 kW, and two-stage systems from around 170 kW. The amount of collector area needed to drive the systems depends to a large extent on the microclimate at the location; this varies enormously in the systems currently installed. Where conditions are favourable, it can be assumed that the specific collector area required is around 2.
Adsorption cooling systems have a slightly lower COP than single-stage absorption systems, at around 0.6, but unlike the latter they can be operated with lower input temperatures, of around 65 °C to 95 °C. Adsorption cooling systems achieve cold temperatures in approximately the 6 °C to 20 °C range. To generate the required thermal solar energy, a specific collector area of an average of roughly 3.4 m²/kW is required. Adsorption cooling systems are already on the market for a low cooling capacity of around 7.
Sorption-supported air conditioning
Like a conventional air-conditioning system, a DEC system is integrated into the building technology concept and thus works with an air inlet and air outlet system. As a first step, the air drawn into the system from the outside is dehumidified and then passed through a heat exchanger integrated into the system. At the same time the outgoing, used air is humidified, thereby cooled adiabatically and then used in the heat exchanger to precool the incoming air. Then the incoming air is cooled further in a conditioning unit to around 16 °C to 20 °C. In contrast to purely adiabatic cooling the combination of sorptive dehumidification, indirect and direct adiabatic cooling generates a significantly lower temperature of incoming air, without notably raising its relative humidity.
Adsorption or absorption can be employed to dehumidify the incoming air. In the case of solid sorbents, a sorption wheel is used. In the case of liquid sorption, a hygroscopic liquid is distributed in a stream of air via packing. To desorb the moisture-laden sorbent again, it has to be heated. For this the previously adiabatically cooled used air is further heated to 65 °C to 95 °C (for example using solar energy) and passed over the moisture-laden sorbent.
The COP of a DEC system is around 0.5 to 1.0. Depending on location, the specific collector area of around 8.2 m² per 1000 m³/h of air flow volume is needed to generate the required desorption energy. With this flow volume a cooling capacity of around 5 kW to 6 kW can be achieved.
Choosing a system
The choice of a closed or an open system depends on the microclimatic conditions at the location of the building and in particular on the level and composition of the loads to be dissipated, as well as on the air-tightness of the building. Thermal loads can be dissipated via core conditioning, ventilated cooling or the removal of warm air and the input of cold air. For core conditioning cold water with a temperature of around 18 °C to 20 °C is required, but this can be provided via a sorption cooling system.
To cool warm incoming air, either sorption cooling systems or DEC systems can be used. Unlike core conditioning or ventilated cooling, where cold water is distributed around the interior of the building, these systems transport cold and warm air, for which extensive duct systems are required, which can lead to problems in particular with refits.
To remove moisture loads (latent loads) it is necessary to treat the air in the room or remove moist indoor air and bring in fresh, dry air. Decentralised dehumidification is possible, by passing the indoor air through a heat exchanger (circulation device) and cooling it down until it passes through the dew point. For this a cold-water temperature of around 6 °C to 9 °C is needed, which can be provided by sorption cooling machines. Incoming and outgoing cold-water pipes are required for this, and a pipe to remove condensate; no air duct system is required. A suitable choice for centralised dehumidification of the air are sorption chillers in combination with an air-conditioning system, or DEC systems. DEC systems can be integrated into the ventilation system of the building in the same way as a conventional air-conditioning system based on compressor technology. An air duct system is required. The volume of air flow should be geared to the exchange of air that is deemed to be hygienically necessary. These volumes should always be checked to ensure they are sufficient to dissipate occurring loads exclusively via a combination of DEC and the ventilation system.
Areas of application
Because of changing meteorological conditions, systems powered only by solar energy cannot produce the cooling output needed 100 per cent of the time. Each year, therefore, there will be a certain number of hours in which the limits of thermal comfort cannot be maintained. Simulations can only give an approximation of how often and when temperatures will rise above comfortable levels. In contrast to systems which operate with solid sorbents, systems using liquid sorbents have the advantage that desorbed sorption agent can be temporarily stored. This makes it possible to bridge periods with lower energy input more easily, without having to switch on a conventional air conditioner based on compression technology.
If sorption cooling systems are to be used in hot, arid areas, a wet cooling tower or similar will have to be used for recooling where outside temperatures are above roughly 32 °C. Which means in turn that water may be needed. DEC systems are basically unsuited for use in hot, arid regions because of the fact that they permanently consume water. A new process, specially developed for hot, arid regions at the Technical University of Darmstadt (Department of Building Design and Technology), is currently in the development phase. It does not consume water and is based on the use of solar-thermal energy. In hot, humid areas sorption cooling systems are unproblematical, as long as recooling of the systems has been secured. DEC systems may need to be combined with a compression-based cooling system.
Sustainability
In comparison to compression-type cooling systems, solar cooling systems have an advantage in terms of the use of primary energy only if the basic minimum of thermal energy required to operate them can be provided through solar energy or available waste heat. In the case of one-stage absorption systems the minimum coverage is 40–75 per cent, for DEC systems, around 55 per cent. To ensure supplies in changing weather conditions, a solar cooling system with a small compression-based cooling system can be added as a back-up. Compensating for insufficient solar energy by burning fossil fuels (above all in systems with a low coefficient of operation) is not sensible in this application from a primary energy consumption point of view.
Cost-effectiveness
Solar cooling systems involve in some cases considerably higher investment and design costs than conventional systems. However, consumption costs are below those of conventional systems. When comparing the systems, alongside investment costs, the ongoing costs of generating cold are a significant factor, so the cost-effectiveness and amortisation of a solar cooling system depend in particular on the future development of energy costs and can only be assessed on a case by case basis.
