Solar systems are being marketed for heating many types of structures, which include greenhouses. Solar energy is absorbed in flat plate collectors where it is ...
Computer Predicted Energy Savings Through Fuel Conservation Systems in Greenhouses C. A. Rotz, R. A. Aldrich, J. W. White ASSOC. MEMBER ASAE
MEMBER ASAE
ABSTRACT OMPUTER programs were developed to model several thermal insulation and solar heating systems with a commercial greenhouse in Pennsylvania. Systems which showed the greatest potential for fuel saving were those which combined insulation and solar heat. With the combination, the fossil fuel requirement for heating the greenhouse was reduced as much as 90 percent of the conventional heating requirement.
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INTRODUCTION In recent years fossil fuels have become more expensive and less reliable for greenhouse heating. Heating expenses form a major portion of greenhouse operating costs and rising fuel prices are pushing this expense even higher. Delivery problems have also made fossil fuels less dependable. Fuel suppliers can no longer guarantee a continuous fuel supply which is unfortunate since a greenhouse is heavily dependent on a reliable fuel supply. If proper temperatures are not maintained in the greenhouse, some crops can be destroyed in a few hours. There is hope, however, for the grower. New products are becoming available which allow the greenhouse industry to be less dependent on these fuels. New products include solar heat and thermal insulation for the greenhouse. Solar heating is a subject which is receiving a lot of attention today. Solar systems are being marketed for heating many types of structures, which include greenhouses. Solar energy is absorbed in flat plate collectors where it is used to heat air or water. The warm air or water is circulated through ducts into the greenhouse where it is stored in a rock bed or an insulated water tank. When heat is needed in the greenhouse, it is drawn from the storage device. Another type of solar heating system, which is being studied for use with greenhouses collects excess heat available inside the structure on sunny days. The heat is stored by circulating the greenhouse air through a rock bed, usually located under the greenhouse benches. At night, when heat is required, the air can be circulated back through the rock bed to be warmed for heating the house. Article was submitted for publication in February 1978, reviewed and approved for publication by the Structures and Environment Division of ASAE in June 1978. Authorized as Paper No. 5428 in the journal series of the Pennsylvania Agricultural Experiment Station. The authors are: C. A. ROTZ, Graduate Assistant, R. A. ALDRICH. Professor, Agricultural Engineering, and J. W. WHITE, Professor, Floriculture, Penn State University, University Park, PA. 362
Another way of reducing fossil fuel dependence is to conserve heat. Because of their current design, greenhouses are poor in retaining heat. Therefore any practice that is used to insulate the house or otherwise reduce heat losses will reduce the fossil fuel requirement. Several solar heating and thermal insulating practices are currently being examined for greenhouse use. Solar systems include air collection and rock bed storage, water collection and storage, and the use of the greenhouse itself for collection and storage of excess heat. Heat conservation systems include the use of thermal insulating blankets which are pulled over the crop at night to reduce heat losses. Another practice is the use of polyethylene or acrylic glazing materials which have better heat retention characteristics than glass. Most of the work with these systems is still in the research and development stage although a few commercial systems are becoming available. There is a neeed to be able to predict the amount of energy these fuel conservation systems can save in any given greenhouse. A study was done by Rotz (1977) to predict the performance of fuel conservation systems with the use of computer models. A commercial size greenhouse was modeled along with several fuel conservation systems under Pennsylvania weather conditions to determine system performance. Both the possible energy savings and the cost benefits were considered in the study. Only the energy savings will be reported in this paper. An economic study is beyond the scope of the present paper and will be reported separately. Two major objectives were set for the study on energy savings. They were to determine the reduction in the fossil fuel energy requirements for greenhouse heating with the use of fuel conservation systems and to find the individual system or combination of systems which could provide the greatest reduction in fossil fuel energy requirements. COMPUTER MODELS Description A computer program was developed which combined several computer models to simulate the performance of conventional and alternative systems for greenhouse heating. A weather model was used to simulate hourly sets of weather conditions for a full year. A greenhouse model was combined with the weather model to calculate a heat balance for a simulated greenhouse under each set of weather conditions. Heating system models were added to simulate the performance of the systems considering both the weather conditions and the heat load
© 1979 American Society of Agricultural Engineers 0001-2351/79/2202-0362$02.00
TRANSACTIONS of the ASAE—1979
START
WEATHER SIMULATION MODEL
ONE HOUR CYCLE
CREENHOUSE HEAT LOAD MODEL
HEATINC SYSTEM MODELS
OUTPUT TEMPERATURES HEAT REQUIREMENTS ELECTRICAL REQUIREMENTS ETC. FIG. 1 Diagram of information flow through computer program.
of the greenhouse. The overall flow of information in the computer program is illustrated in Fig. 1. The model used to simulate weather data was written at The Pennsylvania State University by L. O. Degelman (1974). The weather program was capable of simulating hourly weather conditions for any area of the world. The simulation was made with the use of input data for the location and average monthly weather conditions of the simulated area. Hourly data were derived from the average monthly data by considering both general trends and interrelationships of weather conditions along with a degree of random prediction. A greenhouse model was developed to calculate the heat load of a greenhouse under a given set of weather conditions. It was designed to be versatile so that it could be used to model nearly any size, shape, or type of greenhouse. It was also made as simple as possible to save computation time but yet provide acceptable accuracy in predicting heat requirements. The greenhouse model was developed with the use of basic relationships of heat transfer through structures. These relationships have been outlined in detail in the ASHRAE Handbook of Fundamentals (1972). Walker (1965) was one of the first researchers to extensively apply the relationships to greenhouses. The theoretical basis for this computer model was essentially the same as that developed by Walker. Some modification was made to improve the capability of the model for predicting heat requirements. The model was constructed to function in two major portions or steps. The first step taken was to 1979—TRANSACTIONS of the ASAE
calculate various conditions of the greenhouse environment. The relevant conditions were an overall conductive heat loss coefficient (included conduction, convection, and radiation), the level of solar energy, the temperature and moisture content of the air, the infiltration rate of outside air, and the required amount of ventilation needed to maintain a proper humidity. After these conditions were set, a heat balance was calculated. Heat flows from conduction through the cover, latent and sensible ventilation, and solar insolation were balanced to determine the required or excess amount of heat in the structure. Each of the heat flows was a function of the outside weather conditions and therefore, varied throughout the day. The only heat flows at night were the overall conduction through the cover and a small amount of ventilation due to infiltration. The greenhouse model also had the ability to simulate the different techniques of insulating the greenhouse. These techniques included the use of an insulated cover, or heavily insulated wall on the greenhouse structure. Also included was the possible use of a thermal insulating blanket which was modeled as an additional thermal resistance drawn between the crop and the greenhouse cover at night to reduce heat loss. Several heating system models were also developed. Included were a conventional greenhouse heating system and three basic solar heating systems. Operation of a conventional heating system was simple enough that it did not require computer modeling, however, simulation was required for the remaining three systems. The three modeled solar systems were a solar water system, a solar air system, and an internal greenhouse collection system. The water system included flat solar collectors, an insulated storage tank, an auxiliary heating unit, and other associated equipment. Heat was collected by circulating water from the storage tank through the energy absorbing collectors. When heat was required for the load, water was circulated from the tank to forced convection heat exchangers in the greenhouse. A minimum water temperature was required at the heaters which was provided with the use of an auxiliary heater when the storage temperature was below the required temperature. The modeled solar air system was similar to the solar water system. Major differences were that air was used as the heat transfer fluid and a rock bed was used as the storage media. Heat was collected by circulating air through the collectors and rock bed. When heat was required in the greenhouse, the greenhouse air was warmed by circulating it back through the rock bed. The remaining solar system was the internal greenhouse collection system. It functioned by drawing excess solar heat from the ridge of the greenhouse on a sunny day and storing it in a rock bed. Heat was recovered by circulating the greenhouse air back through the bed, when heat was required in the greenhouse. Validation In order to verify the results obtained from the computer modeling procedure, each of the models 363
TABLE 1. A COMPARISON OF SIMULATED D A T A FOR THE VALIDATION OF THE SOLAR WATER HEATING SYSTEM MODEL Solar water h e a t i n g system
TABLE 2. A COMPARISON OF EXPERIMENTAL AND SIMULATED DATA FOR VALIDATION OF THE SOLAR AIR HEATING SYSTEM MODEL
TRNSYS Difference, (Klein et al.) percent
Total h e a t r e q u i r e m e n t of greenhouse, gigajoule
1728
1710
+1
Auxiliary h e a t requirem e n t , gigajoule
1163
1127
+3
Solar h e a t o b t a i n e d , gigajoule
594
619
-4
Collection efficiency, percent
42
41
+3
Utilization efficiency, percent
40
39
+3
Solar h e a t fraction, percent
33
34
-3
was validated in some manner. Whenever possible, the models were validated by simulating actual systems and comparing simulated data to data obtained from the actual system. An exception had to be made to this procedure. For the solar water heating system, suitable data were not available from a real system to make the comparison. This model was validated by a theoretical method. The weather simulation model was validated by Degelman (1974). He validated his model by comparing simulated monthly, daily, and hourly data to real weather data. The comparison was made using means and standard deviations of average data as well as data on extremes and frequency of occurrence of weather conditions. A good correlation was found between the actual and simulated data. The validity of the greenhouse model was checked by comparing simulated and actual heat requirements for a commercial greenhouse. The selected greenhouse was a 1300 m^ glass house in Bloomsburg, PA. Actual heat requirements for the structure were derived from fuel bills for the month of January, 1977. A computer predicted heat requirement was found by simulating the greenhouse under a month of similar weather data. Simulated and actual heat requirements compared very closely with a difference of only three percent. The solar water heating model was validated by comparing simulated performance data for a particular system to that found with another, better established model. The established model was another computer model developed by Klein et al. (1976) and referred to as TRNSYS. Good agreement was found between the models as shown in Table 1. The model of a solar air heating system was validated by comparing simulated data of a system with that determined experimentally. The experimental system was designed and built at The Pennsylvania State University by Milburn (1977). It consisted of a 37 m^ greenhouse with 17 m^ of external hot air collectors. Data on the system were compared for one day of operation in May 1977. Results are given in Table 2. Some variation did exist in the results, however, the more important comparison was that of collection efficiency which compared very closely. A major difference did exist between the real and simulate systems which should be noted. On the real system, the air movement through the col364
Simulated data
Actual data
Difference, percent
337 195 195 58 7
306 178 132 59 12
+ 10 +10 +48 -1 +38
Available solar energy, megajoule Heat collected, megajoule Heat stored, megajoule Collection efficiency, p e r c e n t Electrical use, megajoule
lection cycle was not a closed loop. The exhaust air from the storage bed was not directly connected to the inlet of the collectors. Exhaust air from the storage was released into the greenhouse, and the greenhouse air was then used as inlet air to the collectors. This created a potential for a low storage efficiency which was reflected by the data. The internal greenhouse collection model was again validated using a small experimental greenhouse at The Pennsylvania State University. Tecza (1977) built and tested the experimental system. Data on system performance was collected for one day in March, 1977. Again good agreement was found between simulated and actual performance as given in Table 3. The more important comparison was that of the solar heat fraction which was the portion of the total heat load which was met with solar energy. These values compared within 2 percent. ENERGY SAVINGS After the computer models were written and validated, they were used to predict energy requirements for greenhouse heating. Conventional and alternative insulating and solar heating systems were studied. Systems Simulated The total system was defined to include the structure and the heating system of the greenhouse. Each of these parts had an influence on the energy requirements. Conventional greenhouse systems were first studied to find conventional requirements. Two systems considered were either a glass or air-separated double polyethylene greenhouse with steam (or hot water) heat. The modeled greenhouses were similar in design. They were a multispan structure with a roof area of 4400, a wall area of 840 and a floor area of 4000 m\ Performance of several alternative heating systems were also studied in conjunction with the convenTABLE 3. A COMPARISON OF EXPERIMENTAL AND SIMULATED D A T A FOR VALIDATION OF THE INTERNAL GREENHOUSE COLLECTION SYSTEM MODEL Simulated data
Actual data
Difference, percent
Total h e a t r e q u i r e m e n t of greenhouse, megajoule
250
268
-6
Auxiliary h e a t r e q u i r e m e n t . megajoule
192
204
-6
Solar h e a t o b t a i n e d . megajoule
49
66
-25
Solar h e a t fraction. percent
23
24
-2
TRANSACTIONS of the ASAE—1979
TABLE 4. COMPUTER SIMULATED ENERGY USE IN HEATING A 4000 m^ COMMERCIAL GREENHOUSE WITH FUEL CONSERVATION SYSTEMS Glass house
System
Energy requirement Gigajoules
Savingst Percent
Double-polyethylene house Energy Savingst requirement Gigajoules Percent
Conventional Heat Oil-fired boiler + unit heaters Insulation Double-acrylic cover* Thin thermal blanket:[: Heavy thermal blanket :|: Double-acrylic cover & thin thermal blanket Double-acrylic cover & heavy thermal blanket Solar Heat
9090
Uninsulated water collector Insulated water collector Insulated air collector Internal greenhouse collector Insulation & Solar Heat Thin thermal blanket & uninsulated water collector Heavy thermal blanket & insulated air collector Double-acrylic cover, heavy thermal blanket & insulated air collector
6908
5636 5381 3781
38 41 58
3686
59
2516
72
6991 5573 5307 7729
23 39 42 15
791
4177 2715
40 61
4939 3636 3506 5559
29 47 49 19
2596
62
950
86
91
* Glass is replaced by double acrylic material. ^Compared to conventional glass greenhouse. :j:Thin thermal blanket, R ^ 1 Heavy thermal blanket, R = 10
tional greenhouse. Their performances were compared to the conventional system to find possible energy savings. The alternative systems included the use of thermal blankets or one of four solar heating systems. Also included for use with a glass house was an insulating, double-acrylic cover for the structure. The solar heating systems studied included two solar water collection systems, a solar air collection system and an internal greenhouse collection system. The external water and air solar systems were all designed with a collector area equal to the floor area of the greenhouse. The water systems used either a well insulated collector or an uninsulated collector. The insulated collector system was designed to represent a system on the market at the present time. The uninsulated collector system used the collector developed by Mears et al. (1977). The rest of the uninsulated system was the same as the insulated water system with a water tank storage and heating provided through forced convection heat exchangers.
except for the internal greenhouse collection system. The performance predicted for this system was found by suppressing the moisture control ventilation of the greenhouse. Over half of the predicted savings was due directly to suppression of this ventilation. This indicated that the actual fuel saving potential INSULATION S i n g l e g l a s s r e p l a c e d by d o u b l e - a c r y l ic cover S i n g l e g l a s s and thin thermal blanket*
S i n g l e g l a s s and heavy t h e r m a 1 b l a n k e t *
SOLAR HEAT Uninsulated water collector
I n s u l a t e d wa t e r collector
Insulated ai collector
RESULTS Each of the alternative fuel conservation systems was simulated in conjunction with both a glass and an air-separated double polyethylene greenhouse. The one exception was the double-acrylic cover which could only be used with a glass house. Similar results were found for the systems when used with either type of greenhouse. A comparison of the energy savings is given in Table 4 and illustrated in Figs. 2, 3, 4. All fuel conservation systems studied did provide a substantial energy savings 1979—TRANSACTIONS of the ASAE
^
I n t e r n a l greenhouse collector
0
20
60
80
100
Reduction in Energy Requirements* (Percent) Thin thermal blanket, R ^ 1 Heavy thermal blanket, R""]:' 10 Compared to conventional glass hou
FIG. 2 Reduced energy use in commercial glass greenhouse heating provided by fuel conservation system. 365
INSULATION Thin thermal blanket*
Double-acrylic alone*
Acrylic cover* and thin blanket
Heavy therma] blanket*
SOLAR HEAT Acrylic cover* and heavy blanket
Uninsulated wate Collector
INSULATION AND SOLAR HEAT Insulated water colle. Acrylic cover*, heavy blanket, and internal collector Acrylic cover*, heavy blanket, and insulated
1 20 Redact
Internal greenhom collector INSULATION AND "SOLAR HRAl' Thin thermal blanket* and uninsulated water collector
Glass is replaced by double a Compared to conventional glas
FIG. 3 Reduced energy use in commercial greenhouse heating provided by several combinations of fuel conservation systems.
for this system was very small. The systems which provided the greatest fuel savings were the thermal blanket systems. A thin blanket reduced energy requirements about 40 percent while a heavy blanket provided about 60 percent reduction. External type solar collection systems also provided energy savings. The insulated collection systems of course performed better than the uninsulated systems. Similar performance was obtained with either the insulated water or air collection systems. Replacement of the glass greenhouse with the double-acrylic house also provided energy savings. The double-acrylic house worked well in combination with other fuel conservation systems to provide very large savings. Solar heating systems and thermal insulating systems worked well together. The largest energy saving was determined by combining a heavy thermal blanket and an insulated solar collection system to any of the greenhouses. The combination provided an 85 to 90 percent reduction in energy requirements when compared to a conventional house. DISCUSSION Values given for this study were obtained by considering an average greenhouse, on an average year, in an average location in Pennsylvania. The values may very considerably when viewed for different greenhouses, different locations, or for various years. The more important results are the relative results. It is more significant to compare the results to fmd which individual systems or combination of systems can save the most energy. These relative results are more consistent for different greenhouses, locations, and years; however, some variation still occurs. The system which showed the least potential for fuel saving was that of internal greenhouse collection of excess solar heat. The simulated results indicate only a small potential for fuel saving. Reasons for the low savings were that the supply and demand for
Heavy thermal blanket* and insulated air collector
REDUCTION
Added to the Compared to ,
!N ANNUAL ENERCY (PERCENT)
RHOUIKLMKNT
ted double polyethylene preenh. ed double polyethylene greenb.o
FIG. 4 Reduction in energy requirements provided by fuel conservation systems with an air-separated double polyethylene greenhouse.
heat occurred at different parts of the year. During the winter months when most of the heat was needed for the greenhouse, little excess heat was available for collection. In the late spring and summer when heat was available, there was little use for it. The systems which appeared to have the most potential were those which combined thermal blankets and external solar collection. This combination allowed a considerable fuel saving. CONCLUSIONS 1 Energy savings were found with the use of three thermal insulation systems and four solar heating systems in heating either glass or air-separated double polyethylene greenhouses. The energy savings varied from a low of 15 percent to a high of 61 percent of the total heat requirement. For the 4000 m^ glass greenhouse, this represents a savings of 1361 to 4193 gigajoules of heat energy. 2 The system which provided the greatest fossil fuel energy saving was a combination of a heavy thermal blanket and external solar collector system. When used with either the acrylic or polyethylene greenhouse, the savings was about 90 percent of the conventional heat requirement. When compared to the conventional glass greenhouse, this represents a savings of 8299 gigajoules of heat energy. References 1 ASHRAE Handbook of Fundamentals. 1972. American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., New York. 2 Degelman, L. O. 1974. A weather simulation model for building energy analysis. Department of Architectural Engineering, The Pennsylvania State University, University Park, PA. 3 Klein, S. A., V^. A. Beckman, and J. A. Duffie. 1976. TRNSYS— a transient simulation program user's manual. Engineering Experi-
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Energy Savings in Greenhouses (Continued
from page
ment Station Report No. 38. Solar Energy Laboratory, University of Wisconsin, Madison, WI. 4 Mears, D. R., W. J. Roberts, J. C. Simpkins, and P. W. Kendall. 1977. The Rutgers solar heating system for greenhouses. ASAE Paper No. 77-4009. ASAE, St. Joseph, MI 49085. 5 Milburn, W. E. and R. A. Aldrich. 1977. Internal/external solar collectors for greenhouse heating. ASAE Paper No. 77-4008. ASAE, St. Joseph, MI 49085. 6 Rotz, C. A. 1977. Computer simulation to predict energy use
1979—TRANSACTIONS of the ASAE
366)
and system costs for greenhouse environmental control. Thesis in Agricultural Engineering. Department of Agricultural Engineering, The Pennsylvania State University, University Park. 7 Tecza, J. 1977. Collection, storage, and use of excess solar heat in a greenhouse. Proposed thesis in Agricultural Engineering. Department of Agricultural Engineering, The Pennsylvania State University, University Park. 8 Walker, J. N. 1965. Predicting temperatures in ventilated greenhouses. TRANSACTIONS of the ASAE 8(3):445-448.
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