Evaluation of Infrared Heating in a Michigan Greenhouse

Transcription

1 Evaluation of Infrared Heating in a Michigan Greenhouse C. Alan Rotz, Royal D. Heins ASSOC. MEMBER ASAE ABSTRACT INFRARED heating has been found to be a practical heating system for a 2790 m^ greenhouse in Michigan. It provided a 31 percent reduction in gas consumption and a 17 percent reduction in costs when compared to a gas boiler system. Uniform temperatures can be maintained and high quality crops can be grown under the alternative infrared heating system. INTRODUCTION Increased energy costs have spurred researchers and growers to develop new ideas for conserving heat in the greenhouse. One idea is infrared radiant (IR) heating, which has been used for several years for heating warehouses and manufacturing plants with up to a 50 percent savings in energy (Roberts-Gordon, 1978). It is now being promoted as an alternative greenhouse heating system. Both researchers and growers have reported very uniform heating and an energy savings with the IR system. A grower in Washington (Youngsman, 1978) reported a 65 percent fuel savings with a gas-fired IR system along with very uniform distribution of heat. He used the system to produce quality crops of bedding plants and foliage. An electric powered infrared system was researched for greenhouse heating by Edwards and Aldrich (1979). They tested the system in a small experimental greenhouse and found an even horizontal temperature distribution and when compared to a fmned-tube, hot water heating system, the IR heated house was found to require 26 percent less energy. Infrared heating has also been tried in double polyethylene covered greenhouses but with less favorable results (Ingratta, 1981). No energy saving over conventionally heated greenhouses of this type are reported. The most probable explanation of this observation is that polyethylene is transparent to infrared radiation (Silverstein, 1976). Radiation reflected from the floor or plant canopy is transmitted through the cover which reduces the effectiveness of IR heating in this type of structure. During the summer of 1979 a major grower in the Central-Michigan area constructed a new greenhouse with a gas-fired infrared radiant heating system.* The greenhouse had a glass roof and double-layer acrylic walls. Article was submitted for publication in January 1981; reviewed and approved for publication by the Structures and Environment Division of ASAE in August Presented as ASAE Paper No Michigan Agricultural Experiment Station Journal Article No The authors are: C. ALAN ROTZ, Assistant Professor, Agricultural Engineering Dept.; and ROYAL D. HEINS, Assistant Professor, Department of Horticulture, Michigan State University, East Lansing, *The authors wish to acknowledge the help of Henry Mast, Jr. of Henry Mast Greenhouse (Byron Center, MI) for providing the facilities and assisting in the collection of data. The new house was located beside two existing houses. One house (conventional house) had an all glass cover and was heated with steam from two gas-fired boilers. The second house was a double-layer polyethylene (double-poly) covered greenhouse heated with gas-fired unit heaters. During the growing season, the three houses were monitored for energy consumption, temperature control, and plant response. Based upon the data collected, an economic evaluation was performed to determine the economic viability of the IR heating system. OBJECTIVES The general objective was to determine the benefits of infrared heating in a Michigan greenhouse when compared to conventional heating systems using boilers or unit heaters in glass or double layer polyethylene greenhouses. The specific objectives were: 1 to measure the natural gas consumed by the three heating systems; 2 to measure the uniformity of soil, leaf, and air temperature; 3 to evaluate the growth of poinsettias, chrysanthemums, and geraniums grown under the three heating systems; 4 to calculate the economic viability of the infrared heating system based upon the measured data. PROCEDURE Greenhouses The IR heated house contained three gutter connected bays, 72.5 m long by 12.8 m wide for a total area of 2790 m^ The IR heating system as a CO-RAY-VACt system which consisted of a 10.2 cm black pipe running parallel to the gutters with 63.3 MJ natural gas burners spaced every 6.0 m along the pipe. Two pipes were run the length of each bay for a total of six pipes and 72 burners. The pipes were spaced equidistant from one another at a height of 4.7 m from the floor. A total of 12 nonaspirated thermostats were used in the house; each controlled two rows of three burners or six burners. Combustion gases from each group of burners were drawn through pipes located under the gutters and were exhausted through vacuum pumps located at the ends of the greenhouse. Plants were grown on benches with a height of around 0.6 m. The other glass covered house was heated with two, natural gas-fired boilers with a capacity of 5300 and 3500 MJ/h. Heat was distributed through finned tubes located under the plant benches. The house consisted of ttrade names are used in this paper solely to provide specific information. Mention of a trade name does not constitute a warranty of the product by Michigan State University or an endorsement of the product to the exclusion of other products not mentioned American Society of Agricultural Engineers /82/ $02.00 TRANSACTIONS of the ASAE-1982

2 six gutter connected bays. The first bay was 6.4 m wide and 45.7 m long. It was followed by three bays 11.0 m by 91.4 m and two bays 12.8 m wide and 91.4 m and 76.2 m long. The six bays provided a total area of 5448 m^ Eleven heating zones were used in the house with a thermostat in each zone. Each of the five larger bays were divided into two zones and the smaller bay was one zone. The double-poly greenhouse consisted of seven bays 6.3 m wide and 58.5 m long for a total area of 2570 m^ The roof was covered with a double layer of air-separated polyethylene and the walls were a double layer of corrugated fiberglass. Heat was provided with gas-fired unit heaters located near the roof at the east end of the house. Warm air was distributed through six fan jets with a polyethylene tube which ran the length of the house. Twelve heaters of 237 MJ/h capacity and two heaters of 265 MJ/h were controlled by one thermostat located in the center of the house. Again plants were grown on benches about.6 m from the floor. Gas Consumption Data were collected in the three greenhouses throughout the heating season. The total flow of gas into the greenhouse complex was measured with a meter provided by the utility company. Individual flows of gas were measured for the IR heated house and the double-poly house heated with unit heaters. By subtracting the gas used by the IR system and the unit heater system from the total consumption, the gas used by the boilers for heating the conventional glass house was determined. The consumption of natural gas was monitored from October 1979 to June 1980 in each greenhouse. The total heating season was divided into three periods. During the fall and spring periods, the same crops were grown in all houses and the same temperatures were maintained. For the winter period different crops were grown and the conventionally heated glass house was set about 4 C warmer than the other houses. Because of the difference in greenhouse construction, modeling was used to determine the portion of energy saved due to the heating system from that due to differences in construction. The conventionally heated glass house was substantially larger in size which resulted in a small ratio of cover area to floor area. The smaller ratio provided a small reduction in the gas consumption per square meter of floor area. The IR heated greenhouse had double-layer acrylic sidewalls which also reduced its gas requirement per square meter of floor area. The heat transfer coefficient for the double-layer acrylic sidewall was about half that of single glass, so it had the same effect as reducing the area of the sidewall by 50 percent. These two factors, by coincidence, canceled each other; therefore, the energy requirements per square meter for heating the two houses were equal when only the structural parameters were considered. This implied that energy consumption differences in each of the glass houses were due entirely to the heating system. Temperature Soil, leaf, and air temperatures were recorded simultaneously in all three houses. Thermocouples were placed in the soil, on the underside of a leaf and in the surrounding air of a representative plant in each house. In the IR house, thermocouples were placed in an aspirated tube to prevent absorption of direct thermal radiation. Temperatures from each of the positions along with the ambient outside temperature were logged on a multi-point recorder. Plant Growth Plant growth was measured for poinsettias, chrysanthemums and geraniums. Most of the plant response data was collected in the fall session. Uniform poinsettia plants potted on the same day and in the same soil medium were placed in each house. Three replications (blocks) of five plants each were monitored in each house. Plant height was recorded regularly throughout the growing season and the final plant height, bract size and bract number were recorded. Plants were treated similarly in all houses. The exact temperatures, however, were not maintained in the three houses. As crops matured, differences in plant growth rate became obvious. Temperatures in each house were adjusted to allow the crop to mature at the proper time for marketing. The energy use measurements, therefore, were for the energy to produce the crop rather than the energy to maintain an exact temperature. Economic Analysis An economic analysis was performed to determine the economic benefit of the IR heating system. All major costs of owning and operating each of the greenhouse structures and heating systems were considered. These included the costs of capital, loan interest, property tax, insurance, repairs, maintenance, and fuel. All costs were viewed over a 10-yr time span with inflation modeled in the analysis. Future costs were determined by multiplying current costs by an inflation factor and then dividing the result by a discount factor to determine present value. Tax deductions were also considered. They included depreciation of capital, deduction of interest payments and deduction of operating costs. For the first year of ownership an investment credit was modeled for all capital expenditures. A total cost for each system was calculated as the sum of all the present value costs of owning and operating the structure and heating system minus the present value of the tax deductions. This total cost figure was used as a basis for comparing the costs of the systems. For more details on the cost model, Rotz (1977) and Rotz (1978) should be consulted. Parameters for the economic evaluation were selected to model a typical grower in this location and were not necessarily the costs of the actual owner. Values for these parameters were gathered through published information and conversations with knowledgeable individuals in the industry. The first group of parameters were those held constant for all systems. These are given in the lower portion of Table 1. The analysis was made to represent owning and operating all systems over the next 10 yrs. All equipment and the structures were considered to be purchased new by a loan. Salvage values of various pieces of equipment were selected to give the percentage of life remaining after it was used for the 10 yr period. Other parameters used in the analysis varied with the type of structure and equipment used. Major parameters included the initial cost, maintenance and repair rate, property tax rate, insurance rate, and electric requirement. Rates for maintenance and repair, property tax, and 1982 TRANSACTIONS of the ASAE 403

3 TABLE 1. PARAMETERS USED IN THE ECONOMIC EVALUATION OF THE VARIOUS GREENHOUSE SYSTEMS. Structure Heating system Parameters Glass Double-poly Glass and acrylic Gas boiler Gas unit heater Infrared Initial cost Salvage rate after 10 years Maintenance and repair rate Property tax rate Insurance rate Electric requirement $86/m^ 50% 2.0% $43/m^ 40% 4.0% 1.3% $90/m^ 50% 1.5% 1.3% $21/m^ 50% 5.0% 1.0o:/mVyr $7.5/m^ 20% 5.0% 3.0<i:/m^/yr $21/m^ 20% 1.0% 1.0<j:/m^/yr Parameters held constant over all systems Period of analysis Life of loan Loan interest rate Loan down payment Natural gas price Natural gas inflation Electric inflation Structure and equipment inflation Discount rate Income tax bracket 10 yr. 10 yr. 12% 20% $.117/m^ 22% 15% 12% 15% 35% insurance were given as the percent of the initial cost which gave a reasonable annual cost. Values for these parameters are given in the upper portion of Table 1. With these parameters, an economic analysis was performed to determine the after tax cost of each of the systems. Comparisons were made to determine how the costs of the IR system compared to those of the more conventional heating systems. RESULTS AND DISCUSSION Energy Savings A summary of the energy data for the three periods is shown in Table 2. Because of the difference in temperature settings during the winter period, the most significant results were those for the fall and spring periods. The data is very consistent between these two periods showing a 31 percent savings with IR heat and a 40 percent savings with the double-poly structure when these systems were compared to the glass house heated with boilers. Table 2 also shows a breakdown of the data for the fall period. The resulting savings of energy in the doublepoly house were consistent from week to week; however. the savings of energy with the IR system was higher during colder periods. This observation is better illustrated in Fig. 1. A significant correlation was found between the heating degree-days for the location and the magnitude of the energy savings. This implies that the IR system was more efficient under cold outside conditions than the conventional system. Energy saved in the IR heated house is primarily due to differences in the heating systems. As stated previously, difference in size and construction of the two glass houses had similar effects which compensated for each other to provide little overall effect on the energy data. Two major factors contributed to the energy savings obtained with the IR heating system. In principle, the system radiates heat to the crop which keeps the heat at the crop level so the attic area remains cooler. Low temperatures near the roof reduce the heat loss through the roof. When averaged over time, temperatures measured in the peak of the IR heated houses were 1 C to 2 C warmer than the average temperatures at plant height. This is in contrast to conventional steam pipe heated houses where temperatures may be 4 C to 6 C warmer at the peak. An IR system also operates at a thermal efficiency near Week Fall* Winter"^ Spring $ TABLE 2. EXPERIMENTALLY MEASURED DATA OF ENERGY CONSUMPTION IN THE THREE TYPES OF GREENHOUSES FOR SELECTED PERIODS DURING THE HEATING SEASON. Date 10/14-10/20 10/21-10/27 10/28-11/03 11/04-11/10 11/11-11/17 11/18-11/24 11/25-12/01 12/02-12/08 10/14-12/08 02/02-04/14 04/14-06/07 Glass house and gas boiler KJ/m^ Glass house and infrared heat KJ/m^ % saving Double-poly house and gas unit heaters KJ/m^ % saving * Thermostat setting maintained air temperature of 17 C in all houses. 1* Thermostat setting maintained approximately 20 C in boiler heated glass house, 17 C in IR heated house and 16 C in double-poly house. X Thermostat setting maintained 22 C in all houses. 404 TFIANSACTIONS of the ASAE 1982

4 o 20H 25h c "> CD CO 20 CD LU tr <.S Correlation: r"^ =.58 Regression Equation: y=.121 x Relative Coldness ( C day/week) FIG. 1 Energy saved with infrared heat when compared to a gas-flred boiler in a glass greenhouse as a function of outside temperature. 90 percent. Therefore, 90 percent of the energy contained in the gas is obtained as heat. This efficiency is gained because exhaust gases are as low as 43 C while in many boilers, exhaust temperatures average between 175 C and 23 C. Since a well-adjusted boiler used in a greenhouse has an efficiency of 70 percent to 75 percent, a 15 percent to 20 percent savings can be obtained through increased efficiency of the IR system. The efficiency of the boiler heating systems used in the conventionally heated greenhouse was measured on two different occasions to determine the actual differences between the systems. The temperature and pressure of the steam, the flow of condensate returned, and the gas consumption were monitored for a period of time. Energy content of the steam was calculated with the use of a steam table, and efficiency was determined by dividing the energy content of the steam by the energy content of the gas consumed. Efficiency of the total boiler system was found to be between 68 percent and 70 percent. The boiler efficiency was also measured by a specialist from the heating industry. It was found to be well adjusted with an efficiency of 80 percent. Differences between measurements were due to losses which occurred in the system outside of the boiler. Energy savings found with the double-poly greenhouse were primarily due to the difference in construction. The air pocket between the layers of poly is a good insulator and is known to significantly reduce heat loss. Another factor which may have contributed to the savings was the protected location of the double-poly house between the two glass houses. This protection from the wind would also reduce heat loss from the greenhouse a small amount, probably about 5 percent of the annual heat requirement. Temperature Control Each of the heating systems was found to provide a uniform temperature throughout the greenhouse. Each system, however, showed a significant fluctuation in TIME (MINUTES) 120 FIG. 2 Temperature fluctuation in a glass greenhouse heated with gasfired infrared radiant heaters. temperature around the thermostat setting. Wide and rapid changes in leaf temperature (8 C to 10 C) occurred under the IR heating system (Fig. 2). The time from the minimum to maximum temperature was approximately 30 min and the entire cycle repeated about once per hour. This heat/cool cycle of about one hour was observed within the outside temperature range of -10 C to 5 C. Significant temperature variation of (4 C to 5 C) occurred in the steam heated glass house in which the total cycle was observed to be about 75 min (Fig. 3). In the double-poly house, relatively small temperature fluctuations occurred (2 C) and the fluctuations were very rapid occurring every 15 min (Fig. 4). These temperature cycles can be explained in part by the thermostats used in the heating systems. The response time for a thermostat is a function of heat transfer to or from the air to the thermostat. Increasing the air flow rate past the thermostat decreases its response time. This occurred in the double-poly house where the thermostat was aspirated. In contrast, the thermostats in the IR heated and steam heated houses were not aspirated. Aspirating these thermostats would have decreased the temperature fluctuations. Uniformity of air temperature across the IR heated TIME (MINUTES) X LEAF + AIR 150 FIG. 3 Temperature fluctuation in a glass greenhouse heated with steam using under-the-bench finned tube heat exchangers TRANSACTIONS of the ASAE 405

5 16- O 14- uj 12- D < S 10- UJ 8- fca fc^vk // \\ //// n 5.5 m above floor \\// (J VVHMM^IW A 3.8 m MW /I \ / x0.9m ^ \ rf + O.b m TIME (MINUTES) TIME (MINUTES) 150 FIG. 4 Temperature fluctuation in a double-polyethylene greenhouse heated with gas-fired unit heaters using aspirated thermostats. greenhouse at plant height was recorded both perpendicular and parallel to the overhead heating lines. Temperatures were very uniform with a maximum difference of 2 C noted between any two points. Generally, the maximum differences were 1 C or less. The winter of was relatively mild in Michigan, with temperatures seldom below 10 C. As a result, the effect of long heating periods on plant growth were not determined. Very cold weather should result in extended heating periods with leaf and flower temperatures of 4 C to 5 C warmer than surrounding air. Flower color may be bleached by such extended warm petal temperatures. In this particular IR system, greenhouse air temperatures at plant height were both cooler and warmer than corresponding set temperatures at plant height (Fig. 2). Air temperatures in a vertical plane were recorded over time in the IR heated house. Temperatures in the greenhouse peaks fluctuated more than those near the plants and on the average stayed a few degrees warmer than temperatures near the plants (Fig. 5 and Table 3). Aspirating the thermostats of the IR system should decrease the temperature fluctuation but may also increase the energy consumption. Air is purged from the FIG. 5 Vertical temperature distribution in a glass greenhouse heated with gas-fired infrared radiant heaters. pipes in the IR system both before and after flame ignition in the pipe. This gas exhaustion practice is necessary for safety reasons but releasing the warm exhaust gas wastes energy. Aspirating the thermostat would increase the frequency of the on/off cycle and, therefore, exhaust more warm greenhouse air. Efficiency would therefore decrease, however, calculations indicated the decrease would probably be less than 1 percent. Plant Response Poinsettia plants grown in the two glass houses were comparable in quality; both were superior to plants produced in the double-poly house. Both bract diameter and the number of large showy bracts were greater in the glass houses. Taller plants were obtained in the IR heated house; however, this may be due to the omission of a cycocel spray (a growth retardant) on plants in this house compared to plants in other houses. A summary of the poinsettia plant growth is presented in Table 4. Comparable plant species were not grown in all three houses during the winter months. A high quality chrysanthemum crop was produced, however, in the IR house for Easter. Seed geraniums were planted in all three houses in mid-march. No significant differences in plant growth were observed between the houses for this crop. Reduced plant quality in the double-poly house can be TABLE 3. VERTICAL AIR TEMPERATURES IN IR HEATED GREENHOUSE. Thermocouple height (m above the floor) Exterior Temperature ( C) Minimum Maximum Difference (aspirated) * *aspirated temperature inside greenhouse was C. TABLE 4. INFLUENCE OF HEATING SYSTEM AND GREENHOUSE COVER ON PLANT HEIGHT, NUMBER AND QUALITY OF BRACTS IN POINSETTIS "ANNETTE HEGG SUPREME." DATA COLLECTED DEC. 4, cover Heating system Plant height (cm) Bract diameter (cm) Number of Bracts Good * Poor* Total Glass Glass Double polyethylene Steam pipe Infrared Unit heater *Good bracts were large and well developed, poor bracts were small, poorly colored or developing under the upper large bracts. 406 TRANSACTIONS of the ASAE 1982

6 TABLE 5. A COMPARISON OF INITIAL COSTS AND AVERAGE ANNUAL COSTS FOR OWNING AND OPERATING THE GREENHOUSE STRUCTURES AND HEATING SYSTEMS*. Greenhouse system Glass structure, gas boiler Glass structure, IR heaters Glass roof, acrylic wall, IR heaters Double-poly structure. gas unit heaters Initial investment. $/m' Operating costt. $/m^ Total costt. $/m^ Saving in total cost, $ 17.3 * Based upon new construction and average energy requirements for Central Michigan area. t Includes loan interest, energy, maintenance, repair, property tax and insurance. X Includes operating costs plus capital costs. Saving in total cost when compared to glass structure with boiler heating system. explained by the reduced light levels. Light intensity measurements were taken with a Li-Cor Quantum meter. Light measurements were made with the sensor base parallel to the floor. Individual measurements were made under the gutter, under the peak, and equidistant between the gutter and peak. This procedure was repeated for all three bays in the IR heated house, under four bays in the double-poly house and under two bays in the conventional house. A mean of all measurements in each house was calculated to represent the average light transmission of the house although the mean is higher than measurements under the gutter and lower than under the peak. On cloudy days, approximately 60 percent of the exterior light was transmitted into both glass houses while only 40 percent was transmitted into the double-poly house. This meant that the glass houses transmitted 40 percent to 50 percent more light on cloudy days than the double-poly house. While not measured, the percentage would be less on sunny days. Economic Analysis Costs of owning and operating each of the systems were evaluated based upon new construction costs. The same size was used for all greenhouse structures to allow a more accurate comparison of systems. The size selected was that of the existing IR heated house, 2790 m^ of floor area. Total energy requirements were based upon those for an average year in the central Michigan area. These were slightly greater than the energy data measured during the heating season. Energy saved with the use of an IR system was set at 30 percent based upon the experimental results. For a double-poly structure, the energy savings was set at 35 percent which was lower than that actually measured but more typical of the savings reported when the greenhouse is not protected by other structures. Acrylic walls when used with a glass roof were assumed to give an additional 7 percent saving of the annual heating requirement over an all-glass cover. A summary of the cost comparisons is shown in Table 5. The IR heating system provides a 17 percent reduction in costs when compared to the conventionally heated glass house. When used in combination with acrylic walls, the costs are reduced an additional small amount. The double-poly structure, because of its low initial cost and lower energy requirement, is less expensive to own and operate than the IR heated house. It provides about a 35 percent savings in total costs when compared to the conventional glass house. The economic analysis has assumed that the crops grown in the three houses are of equal value. In the operation studied, the grower was able to mix the plants and sell them at the same price. If a lower price is set for a lower quality crop grown in the double-poly house, the benefit in costs for the double-poly structure will be reduced and possible eliminated. CONCLUSIONS The infrared heating system has been found to be a practical heating system for a commercial glass covered greenhouse. The following conclusions were reached: 1 Consumption of natural gas was reduced by 31 percent when compared to a gas-fired boiler system operating with an efficiency of 70 percent. 2 Temperatures were uniform throughout the greenhouse at plant height at any time. Large fluctuations in leaf and air temperatures about the thermostat set point were measured through periods of time. 3 Poinsettias, chrysanthemums, and geraniums were equal in quality to those grown in boiler heated glass houses and the poinsettias were superior to those grown in double-poly houses. 4 An infrared system can be owned and operated at a cost 17 percent lower than a boiler system in a glass house and 27 percent higher than unit heaters in a double-poly house. References 1 Bartok, J. W Comparative costs of greenhouse construction. Connecticut Greenhouse Newsletter, No. 98. Cooperative Extension Service, the University of Connecticut. Storrs. 2 Edwards, J. K. and R. A. Aldrich Engineering evaluation of infrared electric radiant heating for greenhouse temperature control. ASAE Paper No ASAE, St. Joseph, MI Ingratta, F Results of research at Vineland. BPI News. Bedding Plants, Inc., P.O. Box 286, Okemos, MI. July, pp Roberts-Gordon Appliance Corp Co-RAY-VAC radiant gas heating system. Promotional material. 44 Central Avenue, Buffalo, NY. 5 Rotz, C. A The economics of energy conservation. Proceedings of the Eleventh International Bedding Plant Conference. Bedding Plants, Inc., P.O. Bos 286, Okemos, MI. pp Rotz, C. A Computer simulation to predict energy use and system costs for greenhouse environmental control. Ph.D. thesis in Agricultural Engineering. The Pennsylvania State University, University Park. 7 Silverstein, S. D Effect of infrared transparency on the heat transfer through windows. A Clarification of the Greenhouse Effect. Science, Vol. No. 193, pp Youngsman, J. E Infrared heat keeps plants warm, fuel bills down at Skagit Gardens. Florist Review, May 10. pp. 118, Youngsman, J. E Infrared heatin'g for greenhouses. Ohio Florists Association Bulletin No Fyffe Course, Columbus, OH TRANSACTIONS of the ASAE 407