Solar Cooling Used for Solar Air Conditioning - A Clean Solution for a Big Problem Stefan Bader Editor Werner Lang Aurora McClain csd Center for Sustainable Development II-Strategies Technology 2 2.10 Solar Cooling for Solar Air Conditioning Solar Cooling Used for Solar Air Conditioning - A Clean Solution for a Big Problem Stefan Bader Based on a presentation by Dr. Jan Cremers Figure 1: Vacuum Tube Collectors Introduction taics convert the heat produced by solar energy into electrical power. This power can be “The global mission, these days, is an used to run a variety of devices which for extensive reduction in the consumption of example produce heat for domestic hot water, fossil energy without any loss in comfort or lighting or indoor temperature control. living standards. An important method to achieve this is the intelligent use of current and Photovoltaics produce electricity, which can be future solar technologies. With this in mind, we used to power other devices, such as are developing and optimizing systems for compression chillers for cooling buildings. architecture and industry to meet the high While using the heat of the sun to cool individual demands.” Philosophy of SolarNext buildings seems counter intuitive, a closer look AG, Germany.1 into solar cooling systems reveals that it might be an efficient way to use the energy received When sustainability is discussed, one of the from the sun. On the one hand, during the time first techniques mentioned is the use of solar that heat is needed the most - during the energy. There are many ways to utilize the winter months - there is a lack of solar energy. energy of the sun when designing a building. However, during the summer, when cooling is The primary and most efficient use of solar needed, there is a great surplus of solar energy is daylighting. In order to use natural energy. The best conclusion to draw from light effectively, the architect has to design these facts is that an efficient way might be to sensibly, allowing the light to enter the building utilize solar energy to additionally generate while avoiding excessive heat gain. Keeping cooling energy. this balance is the difficult part. In the following pages, the advantages of solar In addition to providing natural daylight, solar cooling will be explained by comparing solar energy can also be used through solar thermal cooling with existing compression cooling collectors or photovoltaic panels. Using either technology, while analyzing the efficiency and of these two devices requires a certain amount applicability of each system. of technical equipment. Solar thermal collectors gather solar energy and transfer it to a medium, normally water, that can then be used to heat a certain space. While solar thermal collectors transfer the solar energy directly from one medium to another, photovol- 3 II-Strategies Technology Solar cooling overview fields, some part of the gained energy needs to be used for re-cooling the large system and Historical review most of it will not be necessary in summer (and then might even lead to stagnation Solar cooling technology is actually not a problems), which seems counterproductive. recent invention. It had already been used in Instead, when it is used for cooling, the energy the late 19th century when a solar collector collected in the summer will produce cold at was used to produce ice blocks at the 1878 the time when it is most needed (Figure 5). World Exhibition in Paris by A. Mouchot - the first solar cooling device (Figure 2). In 1892, a A similar situation can be found when looking solar hot water heater was advertised in the at cogeneration processes which also benefit United States of America. Several years later, economically from running continuously. in 1910, one of the first private home solar Therefore, it is interesting to combine standard thermal applications was seen in Pomona cogeneration with thermal cooling: In summer, Valley, USA, where solar collectors were the heat produced can be converted to cold installed on the roof of a private house (Figure directly and thereby prolong the running time 3). MIT took a closer look into solar thermal of the engines (Figure 6). The same applies on a larger scale to district heating. Here again, in Figure 2: World Exhibition 1878 in Paris - A. Mouchot collectors while building the MIT Solar House produced the first manufactured ice block using solar energy in 1939, a research building with an integrated summer it is hard to sell the heat directly, but solar collector roof (Figure 4). Absorption with the addition of a thermal cooling process chillers are not a new invention either. The first the heat can be used all year. applications were developed in the early 20th century for ships and there are some machines Conventional air conditioning that are still running after more than 70 years. Solar cooling has a big potential to restrict the Solar thermal cooling as a valuable add-on enormous amounts of electrical energy feature to conventional applications currently consumed for conventional compres- sion cooling. Electrically driven split-units have Up to now, solar thermal energy has generally their peak loads at the same time during the been used only for domestic hot water and day, when a lot of other electrical consumers heating support. Because of the variation reach their maximum capacity as well. In many through the seasons and the opposite demand countries energy providers have a hard time for cooling in Europe, it appears that it is not providing enough energy for these kinds of ideal to use solar energy for heating alone. air-conditioning machines during their peak The cooling loads of buildings display a times. In addition to electricity demands which parallel energy curve compared to the solar are predominantly met by burning fossil fuels, energy curve throughout the year. It seems these units use ozone-damaging gases and ideal to benefit from solar input while combin- have a leaking range up to 5 - 15 % per year ing solar cooling with a standard solar heating which leads to an additional severe global technology. When solar collectors are used for warming potential (Figure 7). However, the Figure 3: Private home with Solar Thermal application Private(around Home with 1910), Solar PomonaThermal Appli Valley,cation USA (around 1910), Pamona Valley, USA solar heating, large installations are necessary. option to power these units with solar th © SolarNext AG / Hightex Group - Solar Air-Conditioning - Jan Cremers, September 24 , 2008 Because of theclean energylarge for you size of the collector photovoltaic panels could as an alternative to G P P [W] [W] [W] 1 3 2 Jan Dez 0 delta t 8760 [h] 1 Global Radiation 2 Cooling Load Thermal Heat Energy Demand 3 Heating Load Solar Surplus Supply Heat Energy Demand for Cooling in Summer MIT-SolarFigure 4: House, MIT-Solar Research House, Building Research with Integrated Building Solar withCollector integrated Roof (1939), Figure USA 5: Relationship between Solar Radiation and Cooling Figure 6: Combined Heat, Cold and Power: Extension of ©solar SolarNext collector AG / Hightex Group roof - Solar (1939), Air-Conditioning USA - Jan Cremers, September 24th, 2008 Demand clean energy for you Operation Time 4 2.10 Solar Cooling for Solar Air Conditioning solar thermal cooing help to reduce their environmental impact. Competitive advantage of solar thermal Europe 8.1 China 21.5 cooling against conventional compression USA 17.0 cooling Middle Comparing solar cooling with traditionally East 3.7 powered compression cooling reveals some India 2.4 Japan 8.4 significant differences between the two systems, as can be seen in Figures 8 and 9. Solar cooling produces much lower CO 2 Central/South East Asia 7.1 emissions than compression cooling due to its America Africa 1.3 use of the sun as a CO2-free renewable energy 3.9 sources. Although the Coefficient of Perfor- mance (COP), which is defined as the ratio of Australia 0.9 the cooling output to the driving heat required, is lower in the case of solar cooling, the overall CO2 emissions (primary energy related) are significantly lower as it is not the standard national energy mix which has to be taken into account but nearly only the sun: 80% of the Figure 7: Market Situation of Conventional Air-Conditioning Worldwide in 2007 primary energy used for solar cooling comes from the sun rather than from external sources such as fossil fuels, unlike compression cooling, which primarily uses these sources. The higher re-cooling capacities for the solar cooling process are due to the lower COP and the additional heat energy from the collectors Lost Heat: 1.67 kWhth that is brought into the process. The case described in Figures 8 and 9 compare small CO2 Emission: capacity applications (< 10kWh). 369 g/kWh Status on installed solar cooling systems 60 % 0.67 kWh In Europe there are about 200 systems run- el 100 % Compression 1 kWh Cooling ning, which is about 2/3 of all built systems. Primary Energy Chiller th The reality that the combined capacity of 1.91 kWh Lost Energy: COP: 1.5 1.24 kWhth these units is about 15 MW for which they 29 % need a surface of only 30,000 m2 (322,917 ft2) proves the existence of a still small market. 11 % The following cooling technologies were used (Figures 15 and 16): Figure 8: Standard Compression Chiller (< 10 kwth cooling) • 60 % absorption cooling • 11 % adsorption cooling • 25 % DEC systems • 4 % liquid sorption Process of solar cooling Lost Heat: 2.67 kWhth The solar thermal driven cooling technologies CO2 Emission: can be divided into four principal technologies: 83 g/kWh • absorption chillers • adsorption chillers 80 % • open sorption cooling systems (DEC) 100 % chillii® - Technology 1 kWh Cooling • compression chillers ( when driven by Primary Energy 1.67 kWhth COP: 0.6 th 2.1 kWh PVs these are another option for “solar 0.15 kWhel cooling”.