for Simultaneous Heating and Cooling F. Sustainable Energy Centre, University of South Australia Mawson Lakes Boulevard, Mawson Lakes 5095 AUSTRALIA E-mail: Frank.@UniSA.edu.au Abstract The high efficiency of the refrigeration circuit for providing heating or cooling can be increased further if one makes use of both the heating and cooling generated simultaneously. In these cases the heating and cooling energy generated may exceed the energy supplied to the compressor by a factor of more than ten. The University of South Australia has been involved in the development of such a system for a dairy farm. The heat pump is used for combined milk cooling and water heating. The milk from cows is typically at a temperature of 35 C and requires to be cooled to 4 C, whilst the hot water required for washing purposes is to be heated from about 15 C up to 85 C. In order to take advantage of the cheap off-peak electricity tariff, the heat pump generates ice in an ice-bank during the night, which is used for milk cooling during the day. The heat released while the ice is being made is used to produce the daily requirements of hot water. A prototype of the system has been installed on a dairy farm and its performance is currently being monitored. The paper describes the operation of this system and presents some results obtained. Furthermore, it evaluates the performance of the system. 1. Introduction A heat pump makes use of a refrigeration circuit which transfers heat from a source to a sink. Consequently it provides cooling at the heat source and heating at the heat sink. The high efficiency of the refrigeration circuit for providing heating or cooling in comparison with the electrical input to the compressor can be increased further if both the heating and cooling generated are utilised. The Sustainable Energy Centre of the University of South Australia has been involved in the development of this type of system for a dairy farm. Background work which led to this system has been published previously ( & Saman, 1999). The conventional equipment used for cooling milk on a dairy farm is a refrigeration system with an aircooled condenser. In Australia, the refrigeration system either cools the milk directly in the vat during the day, or is used to produce ice in a tank at night which is later used to cool milk during the day. The dairy farm referred to in this paper had the latter system. Water heating on dairy farms is usually achieved by electrical resistance heaters. The use of a heat pump system for combined milk cooling and water heating for dairy farms has been attempted previously. Redding (1987) investigated a system which combines heat-pump heat recovery from the milk refrigeration system with the use of a solar boosted heat pump to meet the balance of the water heating load. Energy usage is reduced by the substitution of heat-pump water heating and the solar input to eliminate all electrical resistance water heating. A full scale experimental unit was designed and installed at a dairy and its behavior and performance were monitored over a full calendar
year. On an annual basis the experimental system used 63% less electrical energy than conventional equipment. Although a combined milk cooling and water heating heat pump system incorporating solar panels is technically feasible, the cost is not justified, and also the system can only be operated during the day when the cost of electricity is high. The Sustainable Energy Centre has investigated numerous systems whereby a heat pump is used to produce ice in a storage tank, which is later used for milk cooling, and the heat released during this process is used to produce the daily requirements of hot water ( & Saman, 1997). The systems are designed to operate at night during the off-peak electricity period, when energy costs are lower and more electrical power is available. One of these systems has been selected for further development. The chosen system has been installed and is currently operating at a dairy farm in South Australia. 2. Description of the heat pump system Milk from cows is normally precooled from about 35 C down to 25 C using water from either a bore, dam or cooling tower, as this is the most energy efficient method of cooling at these temperatures. Thereafter, a refrigeration system needs to be used to cool milk down to 4 C. On the basis of the capacity and operation of the dairy where the heat pump system was installed, the specifications for the system are summarized below: Milk Cooling Capacity: 10,000 L / day Initial temperature of milk: 25 C Final temperature of milk: 4 C Time for cooling: 3 hours Holding temperature: 4 C. Hot Water Capacity: 1,800 L / day Temperature: 85 C. Two main factors are significant in the design of the heat pump system: 1. When the maximum amount of milk is to be cooled daily (10,000 L), more heat is rejected from the milk than is required by the hot water. 2. The coefficient of performance (COP) of the refrigeration system drops rapidly as the temperature difference between the evaporator and condenser increases. The COP is the ratio of cooling energy produced by the refrigeration system and the electrical power supplied to it. When heating is required, the term coefficient of performance for heating (COP h ) is used which is the ratio of heating energy produced by the system and the energy supplied to it. Both factors suggest the need for a heat sink at an intermediate point between the ice tank and hot water. The system selected for the dairy uses an existing underground water tank which collects and stores rain water as the intermediate heat source and sink. The tank has a submersible pump which controls the volume of water in the tank. The volume of water can be set to vary from around 5,000 L up to about 30,000 L. Renewable Energy Transforming Business Refereed Paper 360
The layout of the heat pump system is shown in Figure 1. The heat sources and sinks for this system are the ice tank, water in the underground tank and water in the hot water tank. When the system is first turned on, water in the hot water tank is pumped through the condenser and heat is transferred to it from the ice tank. Once the temperature of the water has reached a preset value, solenoid valves are switched so that water is pumped from, and heat rejected to, the underground water tank. Once sufficient ice has been produced, water is pumped from the underground tank to the evaporator, and from the hot water tank to the condenser, to enable heat transfer from the underground tank to the hot water. This continues until the desired final hot water temperature is obtained. 3. Description of the Monitoring System The data logger selected for monitoring was a DataTaker DT500. This data logger has been connected to a personal computer at the dairy. The computer is used to change settings on the data logger as well as for data storage. Sensors that have been connected to the data logger include eight thermocouples, three thermistors, two pressure transducers, four flow meters and a power transducer. These inputs allow the operating performance and energy usage of the system to be monitored on a continuous basis. Table 1 gives details of the variables measured by the sensors as well as the electrical signal output from each sensor. ice bank pump solenoid valve evaporator compressor throttle valve pump condenser hot water tank underground water tank Figure 1. Layout of the heat pump system. Renewable Energy Transforming Business Refereed Paper 361
Table 1. Data logger Inputs Sensor Type Number Variable Measured Signal Type 1 condenser in (water) thermocouple type T 2 condenser out (water) thermocouple type T Thermocouples 3 evaporator in (water) thermocouple type T 4 evaporator out (water) thermocouple type T 5 hot water in thermocouple type T 6 hot water out thermocouple type T 7 ice bank in (water) thermocouple type T 8 ice bank out (water) thermocouple type T 1 suction temperature (R134a) resistance Thermistors 2 discharge temperature (R134a) resistance 3 condenser out (R134a) resistance Pressure 1 compressor suction 4 20 ma 2 compressor discharge 4 20 ma Power 1 compressor 4 20 ma 1 condenser (water) frequency Flow transducers 2 evaporator (water) frequency 3 ice bank (water) frequency 4 hot water frequency 4. Preliminary results from monitoring Figure 2 shows the instantaneous water temperature at the condenser inlet and outlet during the ice making process. Initially, the heat pump uses water from the hot water tank to cool the condenser. At any point in time, the temperature of the water in the hot water tank is somewhere between that of the water at the inlet and outlet of the condenser. It can be seen in Figure 2 that the temperature of the hot water increases from about 15 C up to about 60 C. At this point the heat pump system switches to the second mode of operation, where it uses water from the underground water tank (at about 30 C) to cool the condenser. Figure 3 shows the corresponding COP h for the heat pump and the electrical power consumed by the compressor. The COP h is initially high but decreases as the water in the hot water tank is heated. There are two reason for this behavior, which both have the effect of increasing the load on the refrigeration system. Firstly, the condenser temperature is increasing with higher water temperatures. Secondly, ice building around the tubes in the ice bank has a greater insulating effect which results in lower refrigerant temperatures in the evaporator. When the heat pump system switches over so that water from the underground tank cools the condenser, the COP h increases as the condenser temperature and hence load on the refrigeration system is reduced. The COP h then gradually decreases as more ice builds around the tubes in the ice bank. The average COP h for heating the water is 4.05, and 3.20 for the remainder of the cycle. Renewable Energy Transforming Business Refereed Paper 362
80 70 ice + hot water to 60 deg. C ice only Temperature (deg. C) 60 50 40 condenser in (water) condenser out (water) 30 20 10 15/5/00 7:40 PM 15/5/00 10:04 PM 16/5/00 12:28 AM 16/5/00 2:52 AM 16/5/00 5:16 AM Time Figure 2: Water temperature at the condenser inlet and outlet (in degrees Celsius) during the ice making process. Compressor Power (kw) and COPh 10 9 8 7 6 5 4 3 2 ice + hot water to 60 deg. C ice only ave COPh = 4.05 ave COPh = 3.20 compressor power COP heat 10 per. Mov. Avg. (COP heat) 1 0 15/5/2000 7:40PM 15/5/2000 10:04PM 16/5/2000 12:28AM 16/5/2000 2:52AM 16/5/2000 5:16AM Time Figure 3: Compressor power (kw) and coefficient of performance of heating (COPh) during the ice making process. Renewable Energy Transforming Business Refereed Paper 363
100 90 Ice + heating water to 50 degrees Celsius. Ice only heating water only to 80 degrees Celsius. 80 Temperature (deg. C) 70 60 50 40 30 condenser in (water) condenser out (water) 20 10 1/5/2000 9:36 PM 2/5/2000 12:00 AM 2/5/2000 2:24 AM 2/5/2000 4:48 AM Time Figure 4: Water temperature at the condenser inlet and outlet (in degrees Celsius) for a complete operating cycle. Compressor Power (kw) and COPh 11 10 9 8 7 6 5 4 3 2 Ice + heating water to ice 50 degrees Celsius. only heating water only to 80 degrees Celsius. ave. COPh = 4.24 ave. COPh = 3.94 ave. COPh = 3.05 compressor power COP heat 15 per. Mov. Avg. (COP heat) 1 0 1/5/2000 9:36 PM 2/5/2000 12:00 AM 2/5/2000 2:24 AM 2/5/2000 4:48 AM Time Figure 5: Compressor power (kw) and coefficient of performance of heating (COPh) for a complete operating cycle. Renewable Energy Transforming Business Refereed Paper 364
Figure 4 shows the water temperature at the condenser inlet and outlet during a complete operating cycle. Initially the heat pump produces ice and heats water in the hot water tank from about 20 C up to about 50 C. In the next mode of operation, water from the underground water tank cools the condenser to complete the production of ice. In the final mode, water from the underground water tank is pumped through the evaporator while water in the hot water tank is used to cool the condenser until the hot water temperature reaches about 80 C. Figure 5 shows the corresponding COP h for the heat pump and the electrical power consumed by the compressor. Up to the point where all the ice is made the same trends are observed as previously in Figure 3. When the system switches into the high temperature water heating mode, the COP h peaks at around 6. The peak occurs because at this point the temperature through the condenser is low (about 30 C) and the temperature in the evaporator is higher than when making ice. As the temperature of the water in the hot water tank increases to 80 C the load increases and so the COP h reduces. The average COP h for heating the water to 50 C is 4.24. During ice only mode the COP h is 3.94. The average COP h for the final heating mode is reduced to 3.05 due to the increase in power consumption as a result of the increase in condenser temperature. The results so far show that the heat pump system is operating according to design specifications. It is able to produce the required amount of ice as well as heat water up to 85 C. The COP of the system is also as expected. 5. Conclusion Monitoring the heat pump system has shown that some coefficient of performance values as high as 6 for heating have been recorded at specific points in the operating cycle, suggesting that the COP for combined heating and cooling was in excess of 10. As the temperature of water in the hot water tank increases, the compressor power increases and the coefficient of performance for heating decreases with a fixed evaporator temperature. Currently the system is showing average COP h values of 4 for heating water from 15 to 50 C, 3.5 for ice only production and 3 for heating water from 50 to 80 C. The monitoring data has shown that the existing water solenoid valves leak some of the water in the hot water tank to the underground water tank, reducing the coefficient of performance of the system during ice only production. Also, at hot water temperatures above 75 C, the efficiency of the heat pump could be improved by increasing the amount of refrigerant in the system. The reason for this is that as the hot water temperature rises so does the pressure of the refrigerant in the condenser which requires more refrigerant for complete condensation. High temperature heat pump systems which utilise both the heating and cooling have potential for many other applications. An example is in commercial buildings where the cooling is used for air conditioning purposes and heating for the production of hot water. Also, such a system can be applied in any industry where both heating and cooling are required in close proximity. 6. Acknowledgments This project is being supported by State Energy Research Advisory Committee of South Australia, University of South Australia, Flaxley Agricultural Centre, Lu've Contardo Pacific Pty Ltd and Frascold Spa. Renewable Energy Transforming Business Refereed Paper 365
7. References F. & Saman. W. (1997), A heat pump system for milk cooling and water heating in a farm: Progress Report No. 2., State Energy Research Advisory Committee of South Australia. F. & Saman W. (1998), Heat pumps for energy efficient simultaneous heating and cooling, Int. J. Renewable Energy, Vol. 15-16:4004, pp. 2296-2299. F. & Saman W. (1999), Heat pump systems for combined cooling and heating, J. Australian Institute of Refrigeration Air Conditioning & Heating, Volume 53, No.3. Redding, G. (1987), A two source heat pump hot water system for a farm dairy: experimental testing and mathematical modeling of an integrated low energy system for farm dairies combining milk-refrigeration heat-recovery and solar evaporator panels. PhD Thesis, University of Melbourne, Australia. Renewable Energy Transforming Business Refereed Paper 366