Capacities for heat pump heating systems for greenhouses in Japan

Transcription

1 Capacities for heat pump heating systems for greenhouses in Japan David R. MEARS, Limi OKUSHIMA, Sadanori SASE,Tadashi TAKAKURA, Hideki MORIYAMA, Shinsuke FURUNO, Masahisa ISHII Corresponding Author: L. Okushima, Abstract Greenhouse heating systems with three types of heat pumps: air source-air supply, water source-air supply and water source-water supply types, are evaluated by hourly calculations based on a simple heat balance model. Cases considered are for the current 4 most typical greenhouses at 4 locations in Japan and for three common night heating set points. From the results, the water source-air supply heat pump could reduce by 30 % the heat pump size compared with the air source heat pump. The water source-water supply heat pump could reduce by % the required heat pump size compared with the air source. The heat pump size requirements for the full supply to the greenhouse heat demand are 4 to 5 times as great as the heat pump size required to meet half the need. The heat pump size requirement to supply 90 % of the full demand is about 2 times the heat pump size required for half the need. Keywords: heat pump, water source, air source, greenhouses, heat demands, heat storage water tank, heat exchanger 1 Introduction Traditionally there has not been a significant utilization of heat pumps, or air conditioners for commercial greenhouse environmental control. An exception has been the use of refrigerant based air conditioning systems for cooling of relatively small greenhouse compartments used for biological containment where there is relatively high risk in quarantine facilities. Design principles for an integrated facility with sections providing various levels of biological containment including compartments that can be sealed and air conditioned for cooling without ventilation are described in Mears et al, 1997 and Mears and Kahn, Containment during periods when cooling is required can also be an advantage when there is a need or desire for carbon dioxide enrichment and issues relating to the design of such a system are discussed in Albright and Henderson, Projects have been undertaken in The Netherlands to develop commercial greenhouse systems that are essentially closed throughout the year using the large aquifer underlying the commercial greenhouse production area as a massive thermal sink for seasonal energy storage for heating and cooling, Bot et al, 2005 and Opdam et al, Heat pumps are used to extract heat from a warm section of the aquifer in winter returning the cooled water to a cool section of the aquifer that will be utilized in water/air heat exchangers for warm weather cooling. The thickness of the aquifer in the region where these projects are implemented is on the order of 100 meters thick providing a substantial thermal storage capacity. The possibility of implementing such a concept for Canadian conditions is discussed by Wong et al, They use a sophisticated finite element model to predict system performance under Canadian weather conditions. In Taiwan Fang, 2009, has developed and implemented a unique application of heat pumps for orchid production. There are significant periods when both heating and cooling of adjacent growing areas are needed where one section needs a cool temperature to promote spike initiation while the other area requires higher temperatures for breeding stock. A heat pump cooling one water storage tank while warming another simultaneously provides appropriate heat sinks for both.

2 In Japan, with increasing fuel cost since 2008, many heat pumps have been installed for greenhouse heating. Cooling and dehumidification by heat pumps has also been introduced to greenhouse production for flowers such as roses and orchids. Most of the heat pumps used in greenhouses in Japan are the air source type, as air can be easily used at anyplace Hayashi, However, in actual practice, the outputs and COPs of air source heat pumps are sometimes less than the catalog specifications, in part because outside air temperatures swing widely, and low outside air temperatures sometimes cause ice frosting on the outside evaporators, reducing performance. There are fewer water source type heat pumps as a suitable water source can t be used everywhere and the initial cost for piping and excavation is higher than for the air source type. In general, the COPs of water source heat pumps are steadier than those of air source heat pumps as water temperatures are steadier than outside air temperatures. There are two options for heat supply systems for air conditioning with heat pumps in greenhouses. One is the hot/cold air supply type and the other is the hot/cold water supply type. The air supply type can supply hot/cold air for air conditioning directly. The water supply type needs an additional water to air heat exchanger for air conditioning. An advantage for the water supply system is the option for heat storage. Those heat pump types each have advantages and disadvantages. Relatively simple modeling systems have been shown useful in designing environmental control systems where mixes of techniques are used for meeting heating or cooling needs or both. Manning and Mears, 1981, used hourly records of weather data matched to corresponding temperatures of the warm water from the condensers of a power plant to design the system to use reject heat for a large commercial greenhouse. The final design included measures to insulate the greenhouse to reduce heating requirements, a mix of floor and air heating systems and the use of a fuel fired backup system to meet peak demands so that the capacity of the waste heat delivery system did not have to meet full peak demands. More recently Both et al, 2005 developed a simple spreadsheet program using representative hourly weather data to investigate different cooling strategies for orchids utilizing multiple layers of shade materials independently controlled to manage heat stress while maintaining growth. A similar approach has been used to investigate the potential performance of several approaches to alternative designs for heating systems including cogeneration, fuel cells and heat pumps, all with storage, Both et al, 2007 and Mears et al, In all cases it is shown that incorporating significant heat storage enables a system with equipment capacity of a relatively small fraction of peak instantaneous demand to meet a very significant portion of the annual heating requirement if backed up with a simple, conventional fuel fired heating system. The advantages of thermal storage have also been demonstrated in simulations of greenhouse spaces attached to residences or other buildings utilizing heat exchangers to capture surplus daytime energy and utilize it during cold nights, Mears and Okushima, This paper considers heating systems with three types of heat pumps: air source-air supply, water source-air supply and water source-water supply types. Cases considered are for the current 4 most typical greenhouses at 4 locations in Japan and for three common night heating set points. The heat requirements were calculated based on the simple heat balance model developed by Both et al, Strategies of greenhouse heating systems for greenhouses in Japan have been considered. 2 Methods The results were calculated using a simulation model based on the one which Both et al. developed (2005). The model is based on steady state, one dimensional heat balance equations for each hour step in a year. The calculation consists of three parts, 1) heat demand calculation of the greenhouse, 2) running capacity calculation of the heat pumps and 3) for the water supply system, the heat storage and discharge calculation of the heat storage water tank and the heat exchangers. Those heat transfer equations are as follows:

3 1) H need = (T in_set -T out )*U-f curt *R in (1) 2a) H output_heatpump = HP size * (c 1 *EST-c 2 )*(ELT-EST)+c 3 *EST+c 4 (air source) (2a) 2b) H output_heatpump =HP size *( c 1 -c 2 *deltat) (water source) (2b) H used_heatpump =Min(H need, H output_heatpump ) (3) 3) H store = HP size *(c 1 -c 2 *(T store - EWT)) (4) H used_store = Min(H need, NO HEX *c HEX *( T store - T in_set )) (5) T t store = T t-1 store +( H store - H used_store )/Mass store -Loss store *( T t-1 store - T out )/ Mass store (6) Notation: T in_set : Night set air temperature inside the greenhouse ( o C) T out : Outside air temperature ( o C) f curt : heat factor of the thermal curtain R in : Solar radiation inside the greenhouse (W m -2 ) HP size : Heat pump size (metric tons = 3.86 kw/ton) H need : Total heat demand of the greenhouses (Wh m -2 ) H output_heatpump : Running capacity of the heat pump (Wh m -2 ) H used : Heat amount (Wh m -2 ) H used_heatpump : Heat supply from the heat pump (Wh m -2 ) H used_store : Heat used from the heat storage (Wh m -2 ) H store : Heat added by the heat pump to the heat storage water tank (Wh m -2 ) c 1, c 2, c 3, c 4 : constants of the heat pump performance EST: Entering water/air source temperature ( o C) ELT: Entering water/air load temperature ( o C) deltat: EST - ELT ( o C) EWT: Entering well, (or pipeline), water source temperature ( o C) T store : Water temperature of the heat storage water tank ( o C) c HEX : constant of the heat exchanger performance = 3164/floorarea NO HEX : the number of the heat exchangers Mass store Heat capacity of water in the heat storage water tank (W m -3 o C -1 ) Loss store : Total heat loss coefficient of the heat storage tank (W m -2 o C -1 ) U: Total heat loss coefficient of the greenhouse (W m -2 o C -1 ) super-script t: time step

4 2.1 Dimensions of the Greenhouses The average greenhouse size in Japan is 477 m 2 and the most popular sizes are m 2. Large greenhouses over 0.5 ha are not very common, only 0.35 % of the total greenhouses area, Japan Greenhouse Horticulture Association, In this study 4 typical size greenhouses were chosen for the calculations. Figure 1 and Table 1 show the dimensions of the 4 typical greenhouses considered. (1) Case 1: 300 m (2) Case 2: 600 m 2 (3) Case3 and 4: 1200 and 2000 m 2 Figure 1. Dimensions of the 4 type greenhouses considered 2.2 U values of the Greenhouses The total heat loss coefficient (U) is one of the significant factors needed to calculate the total heat demands of the greenhouses. We assumed the U of the cover materials is 4.0 W m -2 o C -1 for the PO double layer cover, which is commonly used currently for energy saving greenhouses, and the U of the PO cover with single thermal curtain is 3.0 W m -2 o C -1. Table 1 shows the total heat loss coefficients for the 4 greenhouse types per m 2 of floor area. In this study the structural heat loss and the capacities of the heat pumps, heat exchangers and storages are all modeled based on one unit of floor area, 1m 2.

5 Table 1. Simulation cases Simulation case Case 1 Case 2 Case 3 Case 4 Structure type round super wide multi-span multi-span Floor area(m2) Span No Width (m) Length (m) Eave height (m) Ridge height (m) Covering PO film PO film PO film PO film Area ratio of roof/floor U (gable wall) (W o C -1 m -2 ) U (side wall) (W o C -1 m -2 ) (day) / 3.0 (night with thermal curtain) 4.0 (day) / 3.0 (night with thermal curtain) U (roof) (W o C -1 m -2 ) (day) / 3.0 (night 4.0 (day) / 3.0 (night Total U (W o C -1 m -2 of floor) with thermal curtain) (day)/ (night) 6.72/5.20 with thermal curtain) (day) / (night) 6.25/ Locations and well water temperatures The models were calculated with the hourly weather data of 4 locations, Abashiri, Yamagata, Tokyo and Kagoshima. The characteristics of the weather conditions are shown in Table 2. The annual averaged ground water temperatures were used for the supply well water temperatures for the water source heat pumps. It was presumed the ground water temperature was stable throughout a year. The weather data was derived from METPV-3 data during by NEDO, Table 2. Weather characteristics of the 4 locations Climate zone Location Annual average air temperature ( o C) Annual solar radiation (kwh m -2 year -1 ) Cool Abashiri Moderately-Cool Yamagata Moderately-Warm Tokyo Warm Kagoshima The night set air temperatures The night set air temperatures are 10, 15 and 18 o C, which are shown in Table 3. Those temperatures are the night minimum temperatures of New Zealand spinach, pumpkins, cucumbers, musk melons, eggplants, green peppers and tomatoes, which are typical for greenhouse production in Japan, except pumpkins. Table 3. Minimum night air temperatures, Bartok, 2001 Minimum night air Crop temperature ( o C) 10 New Zealand spinach, pumpkin 15 Cucumber, muskmelon 18 Eggplant, pepper, tomato

6 2.5 The heat Pump performances The three types of heat pumps are air source and air supply (A-A) type, water source and air supply (W-A) type and water source and water supply (W-W) type. The operating parameters for the three heat pump types are derived from the performance specifications of specific commercial units. The model SFYP224A Daikin Co. was selected as an A-A heat pump, model FLHP062 Florida Heat pump Co. as a W-A, and GSW120 ClimateMaster as a W-W heat pump. In each case equations are developed relating the heat transfer in the evaporator and condenser sides to the difference in entering fluid streams. To put these systems on a common basis the coefficients in these equations are scaled so that a nominal metric ton of heating, (3.86 kw), will be delivered when both entering fluids are 16 o C. Thus the equations indicate the differences between the types, but for design of a specific system the actual data from the proposed unit under consideration can be used. The air-to-air heat pump heating output is based on the difference in temperature of the outside air dry bulb and the greenhouse setpoint temperature which is the entering air stream to the condenser. For the water-to-air unit the heating output is determined by the difference in temperature of the geothermal type source and greenhouse setpoint temperature. For the water-to-water system with warm storage only, the heating output is determined by the difference in temperature of the geothermal type source and the temperature in the storage tank. The heat pump output performances were calculated with regression equations from the specification data cited. Figure 2 shows an example of the specification data plots for the W-W system. The plots show the outputs can be described by the linear functions of the differences between the entering source temperature and the leaving load temperatures. The constant values of the equations are shown in Table 4. Note the A-A system requires a four parameter system involving the air temperature as well as the temperature difference of the incoming streams. Figure 2. Performance data of the Heat pump, GSW120

7 Table 4. The regression equations estimated from the specification data for heating type and model Equation Constants heating Air-air, SFYP224A (Daikin Co.) H=(C1*EST-C2)*(ELT-EST)+C3*EST+C4 C1= , C2=0.0325, C3=0.188, C4=9.855 type and model Water-air, FLHP062 (FHP Manufacturing Co.) Equation H=C1*(ELT-EST)+C2 Constants heating C1=0.199, C2= type and model Equation Constants heating Water-water, GSW120 ClimateMaster LSB Industries, Inc.) H=C1*(ELT-EST)+C2 C1= , C2= The heat storage water tank and the heat exchangers It is assumed that the heat storage water tank and the heat exchanger are used to heat the greenhouse inside air for the W-W heat pump system. Each Case 1-4 with the W-W heat pump is calculated with some different water storage sizes. The upper and lower water temperature limits of the water storage tank can be varied for system optimization but are set at 10 o C and 50 o C, respectively for these studies. The initial water temperature in the storage at the first hour in the year is set at 27 o C. The heat exchange from hot water in the storage to the greenhouse inside air is calculated based on the specifications of the Model GLW660 Modine Co. Low Temperature Greenhouse Heating Unit. The heat transfer per o C difference in entering fluid streams is 3164Wh h -1 o C -1 with 218 m 3 min -1 air flow and 152 l min -1 water flow. (Note: there is a similar unit Model GLW330 which will exchange half this amount of heat at half the air flow and 114 l min -1 water flow so it is convenient to model with the variable NO HEX set to half integer values if needed.) 3 Results 3.1 The heat demands of the 4 typical greenhouses at the 4 locations in Japan The total annual heat demands for the four cases at the four locations for each of the three night temperature settings are presented in Table 5.

8 Table 5. The greenhouse heat demands (kwh m -2 year -1 ) Location: Tokyo Set night air temperature Floor area 10 o C 15 o C 18 o C Case m Case m Case m Case m Location: Yamagata Set ni gh t air t emperature Floor area 10 o C 15 o C 18 o C Case m Case m Case m Case m Location: Abashiri Set night air temperature Floor area 10 o C 15 o C 18 o C Case m Case m Case m Case m Location: Kagoshima Set night air temperature Floor area 10 o C 15 o C 18 o C Case m Case m Case m Case m Performance dependence on the greenhouse structures The total heat loss coefficients are 7.58, 6.82, 5.20 and 4.86 W o C -1 m -2 of floor area in nighttime, as shown in Table 1. The U values decrease as the floor areas increase due to the diminishing relative importance of the walls. The ratio of the U value of cases 2, 3 and 4 to the U value of Case 1 are 0.9, 0.7 and 0.6. In response, the ratio of the heat demand per unit floor area of Case 2, 3 and 4 to Case 1 are , and for the three temperature set points used Performance dependence on the locations of the greenhouses The heat demand in Yamagata is from 1.7 to 2.4 times that in Tokyo depending on temperature set point and greenhouse type. The heat demand in Abashiri is from 2.7 to 5.0 times that in Tokyo. The heat demand in Kagoshima is from 0.6 to 0.7 times that in Tokyo. At 18 o C set night air temperature, Yamagata needs 1.7 times as much heat as Tokyo, while at 10 o C set air temperature Yamagata needs 2.4 times the Tokyo requirement Performance dependence on the set air temperatures The night set air temperatures affect the heat demands significantly. Especially in a mild climate area such as Kagoshima and Tokyo, where the heat demands for 18 o C are 4.5 and 3.7 times that for 10 o C, respectively. Meanwhile, in a cooler climate, Yamagata, the heat demand for 18 o C is 2.6 and 2.0 times that for 10 o C. The heating loads for 15 o C in Kagoshima, Tokyo, Yamagata and Abashiri are 2.9, 2.5, 1.9 and 1.6 times those for 10 o C, respectively.

9 3.2 Contribution of the HP size Figure 3 and Table 6 show the annual heat supplied by the heat pumps and the annual heat deficits for Case 3 in which the floor area is 1200 m 2 and with the night set air temperature at 15 o C. For the W-W heat pump system, for any given heat pump size, the heat contribution increases with increasing heat exchange and storage capacities. For the data in this section these capacities were set at levels high enough that further increases relative to the heat pump capacity would not be significant, as discussed in sections 3.4 and 3.5. The W-W heat pump could effectively utilize 1-3 heat exchanger units depending on location and set point temperature. Water storage tank sizes as determined by the heat capacity the heat pump would require about 24 hours for the heat pump to fully charge the storage from minimum to maximum storage temperature with no heat withdrawal during that time. The heat pump size in horsepower is based on 2.8 kw output per horsepower. In Tokyo, if the heat pump could supply all heat demand, the heat pump size of the A-A type, W-A type and W-W type would be 40, 30 and 24 horsepower respectively. The W-W type included 1 heat exchanger and a water storage tank of 62 m 3. If the heat pump could supply half the heat demand, the heat pump sizes of the A-A type, W-A type and W-W type with 1 heat exchanger and 9.8 m 3 water storage would be 12, 9 and 6 horsepower respectively. In Yamagata, if the heat pump could supply all heat demand, the heat pump sizes of the A-A type, W-A type and W-W type would be 55, 39 and 29 horsepower respectively. The W-W type included 2 heat exchangers and a water storage tank of 59 m 3. If the heat pump could supply half the heat demand, the heat pump sizes of A-A type, W-A type and W-W type with 2 heat exchangers and 12 m 3 water storage would be 16, 13 and 10 horsepower respectively. In Abashiri, if the heat pump could supply all heat demand, the heat pump sizes of the A-A type, W-A type and W-W type would be 110, 70 and 44 horsepower respectively. The W-W type included 3 heat exchangers and a water storage tank of 98. m 3. If the heat pump could supply half the heat demand, the heat pump sizes of the A-A type, W-A type and W-W type with 3 heat exchangers and 23 m 3 water storage would be 26, 20 and 15 horsepower respectively. In Kagoshima, if the heat pump could supply all heat demand, the heat pump size of the A-A type, W-A type and W-W type would be 40, 30 and 24 horsepower respectively. The W-W type included 1 heat exchanger and a water storage tank of 50 m 3. If the heat pump could supply half the heat demand, the heat pump sizes of the A-A type, W-A type and W-W type with 1 heat exchanger and 9.8 m 3 water storage would be 10, 8 and 5 horsepower respectively.

10 (1) Tokyo (2) Yamagata

11 (3) Abashiri (4) Kagoshima Figure 3. The annual heat supplied by the heat pumps and the annual heat deficits in Case 3 (1200 m 2 ) with the night set air temperature 15 o C (The percentages are 50, 90, and 100 % of total heat requirement.)

12 The water source system could downsize the heat pump sizes -6, -7, -10 and -26 horsepower in Tokyo, Kagoshima, Yamagata and Abashiri, respectively compared with the air source for full heat supply to the greenhouse (Case 3) to meet heat demand, and -3, -3, -3 and -5 horsepower for half of the heat supply. From the results, the water source heat pump could reduce by 30 % the heat pump size compared with the air source heat pump. The W-W heat pump could reduce by 40-50% the required heat pump size compared with the air source. The reduction of required heat pump capacity could reduce the use of electric power and heat pump investment, although additional cost would be needed for the heat exchangers and a heat storage tank. Furthermore, there is the possibility, not yet investigated, with storage, to improve the control strategy to concentrate the use of the heat pump to off-peak hours when heat demand does not require 24 hour per day heat pump operation. The heat pump size requirements for the full supply to the greenhouse heat demand are 4 to 5 times as great as the heat pump size required to meet half the need. The heat pump size requirement to supply 90 % of the full demand is about 2 times the heat pump size required for half the need. Table 6. Heat pump size (horsepower) to supply 100 %, 90 % and 50 % of the heat demand in Case 3 with 15 o C set night air temperature For 100 % heat demand For 90 % heat demand For 50 % heat demand W-W W-A A-A W-W W-A A-A W-W W-A A-A type type type type type type type type type Tokyo Yamagata Abashiri Kagoshima Backup boiler sizes Figure 4 shows the hourly heat demand and supplies using A-A, W-A heat pumps and W-W heat pump with the heat exchanger and the heat storage in Case 3 and the night set air temperature is 15 o C. The heat pump sizes for each location are the size required for the W-W heat pump to meet 90% of the full heat demand. In each graph the hourly results for the system performances are sorted from the highest heat demand hour to the lowest for plotting. In Kagoshima, greenhouse heating is necessary for about 2000 hours in the yearly total of 8760 hours. The maximum hourly heat demand is about 75 Wh h -1 m -2, and the capacities of the backup boiler are 60 kw (=50 Wh h -1 m -2 x 1200 m 2 ) for the A-A heat pump and 48 kw for the W-A and W-W heat pumps. The heat supply from the heat exchanger and the storage with the W-W heat pump varies hourly as the water temperature in the storage is changing. Then the capacity of the backup boiler required for the W-W system is not different from that for the W-A. However, the W-W needs much less annual back up heat than the W-A. The W-W system reduces the backup heat 70 % compared to the W-A with the same size heat pump unit. In Tokyo, heating is necessary for about 2500 hours in a year. The maximum hourly heat demand is about 80 Wh h -1 m -2, and the capacities of the backup heater are 72 kw (=60 Wh h -1 m -2 x 1200 m 2 ) for the A-A heat pump and 60 kw for the W-A and W-W heat pumps. The W-W system reduces the backup heat 60 % compared to the W-A. In Yamagata, heating is necessary for about 3500 hours in a year. The maximum hourly heat demand is about 110 Wh h -1 m -2, and the capacities of the backup heater are 96 kw (=80 Wh h -1 m -2 x 1200 m 2 ) for the A-A heat pump and 72 kw for the W-A and the W-W. The W-W system reduces the backup heat 60 % compared to the W-A.

13 In Abashiri, heating is necessary for more than half the year, about 4500 hours. The maximum hourly heat demand is about 160 Wh h -1 m -2, and the capacities of the backup heater are 156 kw (=130 Wh h -1 m -2 x 1200 m 2 ) for A-A type heat pump and 120 kw for the W-A and the W-W. The W-W system reduces the backup heat 50 % compared to the W-A. It is significant to note that in each location the W-W system does contribute more than the others for all weather conditions. However, in the coldest periods the increases over the W-A system are not as great as in slightly warmer weather. This is related to the reduction in temperature in the warm storage during extended periods of the coldest weather. For each location there is an outdoor weather condition such that when the weather is warmer than that, the W-W system can provide all of the requirements. In warmer weather it is likely heat pump system efficiency could be improved somewhat if the control strategy were to reduce the maximum storage temperature to a level closer to the lower limit the heat exchanger needs to meet the demand. (1) Kagoshima (The heat pump size: 11 horsepower) (2) Tokyo (The heat pump size: 13 horsepower)

14 (3) Yamagata (The heat pump size: 20 horsepower) (4) Abashiri (The heat pump size: 30 horsepower) Figure 4. Hourly heat demand and heat supplies by the heat pumps in Case 3 (1200 m 2 ) and the night set air temperature 15 o C 3.4 Effect of the heat storage water tank capacity Figure 5 shows that the heat supply by the W-W heat pump increases as the storage tank volume increases and with increasing heat exchanger capacity. The rate of the heat supply increases significantly as storage increases from 6.4 m 3 to 36.7 m 3 but further increases in system performance are insignificant beyond that. The 36.7 m 3 water tank is the size which the W-W type heat pump can fully charge from the minimum to maximum storage temperature limits in about 24 hours with no heat withdrawal during that time. When the number of heat exchanger units was 0.5 for this situation, the effect of the size of the storage tank was small as at that level heat exchanger capacity is the limiting factor.

15 Figure 5. Heat supply by the W-W type heat pump (28 horsepower) with heat exchanger numbers = versus the water heat storage tank capacity for Case 4 (2000 m 2 ) and the night set air temperatures 15 o C in Abashiri. 3.5 Effect of heat exchanger capacity Figure 6 shows that the heat supply by the W-W type heat pump increases as the number of heat exchanger units increases. At any water storage capacity, ( m 3 ), the increased rate of the heat supply with the number of heat exchangers from 0.5 to 2 was larger than that with more than 2 units. This means that 2 units have good cost performance for this Case 4, more than 2 units did not increase the heat supply significantly. Figure 6. Heat supply by the W-W heat pump (28 horsepower) with the water storage tank ( m 3 ) versus the number of heat exchanger units in Case 4 (2000 m 2 ) and the night set air temperatures 15 o C in Abashiri. 4 Discussion In Japan, the annual average air temperatures range widely, such as from 7 o C in Abashiri to 19 o C in Kagoshima. The heat demands of greenhouses range from 60 kwh m -2 year -1 in Kagoshima to 310 kwh m -2 year -1 in Abashiri for a greenhouse of type Case 3 with 15 o C night set air temperature. The difference of annual heat demand is about a factor of 5 based on location. The night set air temperature also affects the heat demand strongly. Therefore, the design and control strategy of the heating system should be carefully matched to any location and crop requirement in Japan.

16 The optimum heat pump size ranges widely depending on the heat pump types, A-A, W-A and W-W and with the heat storage and the heat exchanger capacities. The suitable heat storage water tank is the size at which the heat pump could increase storage temperature from minimum to maximum limits in about 24 hours with no heat withdrawal. The number of the heat exchangers of the type utilized in this study was one unit per 1000 m 2 floor area for the highest heat demand situation. The calculations in this paper can provide a rough guide to plan heating systems with a heat pump. For example, in Tokyo, the backup heat requirements with the W-A heat pump system and W-W system were 30 and 10 kwh m -2 year -1, respectively when the both heat pump sizes were 13 horsepower in Figure 3. The difference of the backup heat requirements between the W-A and the W-W, of 20 kwh m -2 year -1 reflects the saving added by the heat storage. The fuel oil saving would be yen year -1 roughly when the greenhouse was 1200 m 2, the fuel oil cost was 80 yen l -1 and the heat content per fuel unit was assumed10 kw l -1. The initial costs of the backup boiler, heat storage water tank and the heat exchange unit and the running costs would be significant factors to install the W-W heat pump system. Actual designs will need more detailed and accurate calculations. This modeling system can be used to consider a range of possible situations including extension of the use of the equipment for cooling to keep the greenhouse closed for increased portions of the day and the potential for systems with storage to provide some capacity to survive limited periods of power outage. Notes: This research was promoted by No of Research and development projects for application in promoting new policy of Agriculture Forestry and Fisheries. We thank Daikin Co. for providing the specifications data of Model SFYP224A. The study does not make recommendations for the particular products discussed here. They were used for the calculations as model cases. References Model FLHP GT062 Specifications, FHP Manufacturing Co., Fort Lauderdale, FL. Model GLW660 Specifications, Modine Manufacturing Co., Racine, WI. Model GSW120 Specifications, ClimateMaster, LSB Industries, Inc. Oklahoma City, OK. Model SFYP224A Daikin ref, DAIKIN INDUSTRIES,LTD Albright, L.D. and H.I. Henderson Air conditioning greenhouses to increase effectiveness of carbon dioxide enrichment. ASAE paper No ASAE, 2950 Niles Road, St. Joseph, MI , USA. 15 pp. Bartok rev. Energy Conservation for Commercial Greenhouses. NIRAES-3, 152 Riley-Robb Hall, Ithaca, NY , USA. 84pp.

17 Bot, G., van de Braak, N., Challa, H., Hemming, S., Rieswijk, T.H., van Straten, G. and Verlodt, I The solar greenhouse: state of the art in energy saving and sustainable energy supply. Acta Hort. (ISHS) 691: Both, A.J., E. Reiss, D.R. Mears, and W. Fang Designing environmental control for greenhouses: Orchid production as example. Acta Horticulturae 691(2): Both, A.J., D.R. Mears, T.O. Manning, E. Reiss, and P.P. Ling Evaluating energy savings strategies using heat pumps and energy storage for greenhouses. ASAE Paper No , Minneapolis, Minnesota, June Fang, W Use of heat pumps for energy-saving in protected cultivation. Symposium of controlled closed systems toward efficient bioproduction. Matsudo, Chiba, Japan. Manning, T.O. and D.R. Mears Computer aided design of a greenhouse waste heat utilization system. Energy in Agriculture 1:5-20. Mears, D.R., A.J. Both, L. Okushima, S. Sase, M. Ishii and H. Moriyama Some alternatives to burning fuels for greenhouse heating. J. Agric. Meteorol. 65(3): Mears, D.R., L. Okushima Energy focus for sustainable agriculture in metropolitan areas. Agricultural Structures and Environment Special Seminar No. 38. NIRE. Tokyo. March 24, Mears, D.R. and R.P. Kahn Concepts and new designs for quarantine greenhouses. Published in Kahn, R.P., S.B. Mathur et al. Exotic pest and pathogen exclusion: Containment facilities and safeguards for imported seeds, plants and biological control organisms. APS Press, St. Paul, MN, Chapter 11, p Mears, D.R., T.O. Manning and R.P. Kahn New concepts and designs for quarantine greenhouses. Paper No. 9724, Northeast Agricultural and Biological Engineering Conference, July 14-16, College Park, MD. (P ) Opdam, J., Schoonderbeek, G., Heller, E., & De Gelder, A. (2005). Closed greenhouse: A starting point for sustainable entrepreneurship in horticulture. ISHS: Acta Horticulturae, 691, pp Wong, W., L. McClung, D. McClenahan, A. Snijders and J. Thornton. (2007). The application of aquifer thermal energy storage in the Canadian greenhouse industry. GreenSys Conference, Quebec City, Canada, June Hayashi, M. ed Effective utilization of heat pumps for greenhouses - Energy conservation and multilateral uses -. Agriculture Electricity Association, Iwamoto, Chiyoda, Tokyo , JPN. 140 pp. (in Japanese) Agricultural Production, Bureau Ministry of Agriculture, Forestry and FisheriesJapan Greenhouse Horticulture Association. Installation report of glass houses and plastic houses for horticulture. Japan Greenhouse Horticulture Association, Higashi-nihonnbashi, Chuo, TTokyo , JPN. 197pp. (in Japanese)