Thermal Energy Storage for More Efficient Domestic Appliances Halime PAKSOY, Selma YILMAZ, Ozgul GOK, Metin O. YILMAZ, 2Muhsin MAZMA, Hunay EVLIYA Çukurova University 01130 Balcalı Adana-Turkey [email protected] , [email protected] , [email protected] , [email protected].,tr [email protected] 2TÜB İTAK Marmara Research Center, Gebze-KOCAEL İ, TURKEY [email protected] ABSTRACT Increasing energy efficiency of domestic appliances will decrease energy consumption in residential sector. Using waste heat given off while appliance is working is one way of increasing energy efficiency. Sources of waste heat and temperature levels show differences in different domestic appliances. In this paper, increasing energy efficiency of dishwashers and refrigerators through latent heat storage in phase change materials (PCM) is discussed. PCMs are developed for this purpose. Results for a case study of waste heat recovery in dishwashers showed that maximum temperature increase in the inlet temperature of the second washing cycle was 13.4 oC. Keywords : Domestic appliances, energy efficiency, latent heat storage INTRODUCTION Thermal energy storage (TES) systems provide alternative solutions to benefit from renewable energy and waste heat. Thermal energy storage is realized as a result of the change in internal energy of a material. One or combination of the following heats is utilized in TES systems: sensible, latent and/or chemical reaction. Change in temperature of a material is used for sensible heat storage. Heat accompanying a phase change of the material is used for latent heat storage. Thermal energy may also be stored as the energy of a chemical compound, and energy can be repeatedly stored and released in the same materials by reversible chemical reactions. This generally involves a reversible chemical reaction, absorption, adsorption or a hydration process. [Abhat, 1983]. Latent heat storage system with phase change material (PCM) is preferred for short term applications of heating and cooling. High storage capacity and isothermal behavior of PCMs make them favorable choices [Zalba et al., 2003; Sharma and et al., 2005]. Organic and inorganic materials can be used as PCM for the application of the latent heat storage. Although inorganic PCMs have higher melting enthalpies per volume than organic PCMs, they are corrosive. Moreover, organic PCMs do not show supercooling. The choice of PCM is made considering thermal, mechanical and economical aspects [Mehling and Cabeza, 2008]. Among the applications that benefit from PCMs are passive heating/cooling, enhancing stratification of solar hot water tanks, solar collectors, waste heat recovery in industry and appliances, transportation of temperature sensitive products, concentrated solar power plants [Paksoy, 2007]. In this paper, increasing energy efficiency of dishwashers and refrigerators through latent heat storage in PCMs is discussed. PCMs are developed for this purpose. Results for a case study of waste heat recovery in dishwashers are also given. ENERGY CONSUMPTION IN DOMESTIC APPLIANCES Significant share of electricity consumption in residential sector is used for domestic appliances. The residential sector, following industry, consumes 37% of the electricity produced in Turkey. Refrigerators has the largest share (31.1 %) in this consumption, followed by washing mashines (8.5 %) and dishwashers (3.5%) [refererans]. The appliances are rated between A (highest) and G standards according to energy consumption. Significant amount of energy can be conserved when more energy efficient appliances are used. A study made in Turkey shows that CO2 emissions that could be cleaned by 120 milion trees will be avoided in 10 years if all of the refrigerators bought are above A standard. Yearly energy conservation when domestic appliances of A standard are used is 20% [www.tutev.org.tr/enerji_panel/Arcelik_sunum]. In 1995, electricity consumption in Europe for domestic appliances was 264 TWh. This value is equivalent to 130 million ton CO2 for a fossil fuel power plant. In 2005 energy consumption dropped to 230 TWh and 17 million ton CO2 gas was avoided. For washing machines energy efficiency has been increased from 38% to 76% between 1994 and 2001. [www.ceced.org] RECOVERING WASTE HEAT IN DOMESTIC APPPLIANCES The latent heat storage can be used to recover waste heat in domestic appliances. Phase change materials which can melt at the temperatures of waste heat are required for this purpose. There are various ways to increase energy efficiency of domestic appliances. One of them is using waste heat given off while appliance is working. Sources of waste heat and temperature levels show differences in different domestic appliances: o Washing machines and dishwashers: 30 – 90 ºC o Refrigerators and deep-freezers:-18 - +8 ºC o Ovens: >100ºC In washing machines and dishwashers waste heat given off at the end of the first washing cycle can be used to pre-heat the water in the second washing cycle. In refrigerators, there are different alternatives for placing PCM storage unit in the vapor compression cycle in order to increase COP. In addition, waste heat which is released from the foods in the cabin of the refrigerator can be stored in PCM to maintain homogeneous temperature distribution in the cabin. Ovens release considerable amount of heat at moderate to high temperatures. Heat loss from the ovens can be controlled by using PCMs and cooking time can be optimized. There are few studies on using phase changing materials for domestic appliances in literature. In the patent by Longardner and et al. [1993], the design of a PCM heat exchanger for dishwasher and washing machine applications was published. This was a coaxial heat exchanger with two cylindrical chambers inside one another. The inner chamber with PCM was intended to store waste heat from the fluid in the external chamber [Longardner and et al., 1993]. In another patent, PCM was used to improve the drying performance of dishwasher [Werner, 2000]. Waste heat of the moist hot air in the drying process is recovered by a heat exchanger with PCM in this patent [Werner, 2000]. Azzouz and et al. proposed to increase the energy efficiency of the refrigerator using PCM. It was shown that evaporator temperature can be controlled to increase heat transfer by adding a PCM storage unit near evaporator of refrigerator [Azzouz and et al., 2005, 2008]. Wang and et al. claimed that the COP of the refrigerator improved 4%-7% by using PCM near condenser of refrigerator [Wang and et al., 2007]. LATENT HEAT STORAGE FOR DOMESTIC APPLIANCES Dishwashers Four different PCMs are prepared for dish washer waste heat recovery: PCM-A (melting point as given by manufacturer: 42 – 44 oC), PCM-B (melting point as given by manufacturer: 43 o C), PCM-C (melting point as given by manufacturer: 35 o C), and PCM-D (melting point as given by manufacturer: 32 o C). Thermal stabilities of the PCMs were determined with 1000 thermal cycling tests. In an experimental set-up waste heat recovery tests are carried out. Figure 1 shows the inlet temperature of the second washing cycle was increased from 23.0ºC to 36.4ºC for PCM-A. This temperature difference, ∆T of 13.4ºC represents the degree of pre-heating accomplished in the second washing cycle. Hence energy consumption for heating in second washing cycle will be less and the energy efficiency of the dishwasher will be increased. The corresponding increase in energy efficiency is calculated as 22 %. 55,00 50,00 Tin Tout 45,00 C) o 40,00 35,00 Temperature ( Temperature 30,00 ∆T 25,00 20,00 0 500 1000 1500 2000 Time (s) Figure 1 . Measured inlet (T in ) and outlet temperature (T out ) of the TES unit during storage and recovery experiments for PCM-A at heat storage temperature of 52ºC Table 1 shows the temperature differences obtained in the storage and recovery experiments of the other PCMs tested at two different storage temperatures. The maximum temperature difference of 13.4oC was measured for PCM-A at 52ºC storage temperature. Table 1. Temperature differences measured in Storage and Recovery Experiments for PCMs Storage Temperature 52 °°°C Storage Temperature 42 °°°C ∆∆∆ °°° ∆∆∆ °°° T( C) T( C) PCM-A 13.4 9.8 PCM-B 12.2 7.8 PCM-C 8.8 7.6 PCM-D 9.2 8.5 The corresponding calculated increases in energy efficiency of the dishwasher for different PCMs tested at storage temperature of 52ºC are shown in Figure 2. The increase in energy efficiencies was between 22 % and 9 %. 25,00 21,61 20,00 18,51 15,00 10,34 10,00 9,18 % Increasein EnergyEfficiency 5,00 0,00 PCM-A PCM-B PCM-D PCM-C Phase Change Material (PCM) Figure 2. Calculated increase in energy efficiency of dishwasher with different PCMs Refrigerators Genarally there are two compartments depending on their purpose of usage in the refrigerators; one working at a temperature interval of (–18) – (-25) °C, and the other at (+2) – (+8) °C. Depending on the temperature set interval, when the temperature goes above the temperature set limit, cooling system of the refrigerator starts. If the cooling system starts and stops for small time intervals, energy consumption of the system increases. Moreover the longer the stand-by duration, which is the period when the cooling system is not working, the less will be the energy consumption of the refrigerator. Increasing stand-by duration depends on keeping the desired temperature in the refrigerator for a longer time. By incorporating PCM with appropriate melting/freezing range in the refrigerator, upon any increase in temperature due to various reasons, PCM will melt and the temperature will be kept around the desired level. Hence there will be less demand for the cooling system operation and energy consumption will decrease. PCM can also be used together with the insulation material to decrease the heat losses.