| Considerations for Energy Management of Greenhouse
Heating and Cooling Southern Greenhouse Vegetable Growers Association Conference Shreveport, LA July 19 - 20, 2002 Dr. Gene A. Giacomelli Controlled Environment Agricultural Center -Agricultural & Biosystems Engineering Department, Universidad de Arizona, USA Introduction to Environmental Control for Energy Conservation Energy conservation strategies begin with an understanding and evaluation of heating and cooling systems, which are the primary consumers of greenhouse operational energy expenses. Once these systems are optimized then conservation techniques, alternative energy sources, or alternative management techniques should be considered for reducing energy costs. The controlled environment includes the greenhouse, or other structure, and its environmental control systems that are implemented to obtain the desired climate in order to produce a quality crop within a predictable and repeatable time schedule. The controlled environment must be constructed to be compatible with the needs of the nutrient delivery system and the plant culture technique. The best environmental control systems are not only effective in providing the desire plant environment, but they are designed and constructed to be unobtrusive within the greenhouse system. Environmental control for heating and cooling uniformity is a very important design consideration to maintain desired environmental setpoint conditions. However, the distribution of heat is difficult, and a uniformly heated environment may not result. Non-uniform environments cause differential plant growth rates, potential disease problems, unpredictable results with nutrition or hormonal application, and generally a more difficult plant production system to manage. For the most effective and uniform cooling and heating, the rows of plants should be arranged in the direction parallel with the ridge or gutters of the greenhouse structure. For ventilation, this assumes that the ventilation system (fans and air inlets) would be located on the end walls (perpendicular to the direction of the gutters). Should airflow be restricted and non-uniform, then the ventilation system cannot effectively cool the plant, nor provide for sufficient air exchange for humidity reduction (disease control) and replenishing carbon dioxide. Evaporative cooling systems, whether pad and fan, or high-pressure fog, are highly dependent upon effective and uniform ventilation, as well. Similarly, row orientation can improve air movement and more effective heating of each plant. A taller greenhouse is better for improved climate uniformity. In gutter-connected greenhouses, a minimum of 3 m (10 ft) from floor surface to gutter, plus an additional 1.2 m (4 ft) from the gutter up to the ridge, is desirable. Tall greenhouses provide a large internal air volume, which reduces rapid changes of the greenhouse climate caused by the natural daily fluctuations of the outside environmental conditions. In addition a tall greenhouse provides sufficient space required for other greenhouse systems such as: an energy blanket or shade cloth, supplemental lights, raised benches (reducing usable height to overhead systems), irrigation boom, overhead misting systems, tall crops such as tomatoes, or hanging basket plants. Ventilation is the controlled exchange of ambient air with the conditioned atmosphere within the greenhouse. The primary purpose is to reduce the air temperature, and therefore ventilation is usually controlled with an air temperature thermostat. Other environmental parameters which are directly influenced by the ventilation process are the concentration of gasses within the greenhouse air, including carbon dioxide (CO2), oxygen, and water vapor. Carbon dioxide is the most critical of the gasses because of its direct relationship to the photosynthetic processes within the green plant. An exchange of oxygen for carbon dioxide at the surface of the leaf is controlled by the diffusion process. The rate of diffusion can be drastically increased with air movement around the leaf. This benefit can be a direct result of adequate ventilation. Undesirable concentrations of gasses such as ethylene and carbon monoxide will be prevented as a result of adequate ventilation. Finally, atmospheric moisture content, typically measured as relative humidity can at times be reduced by proper ventilation, although it may be necessary to provide supplemental heating to offset excessive air cooling. The control of the ventilation system must be linked with the control of the heating system. The engineer or designer must insure that ventilation will not occur when the heating system is activated. The techniques of ventilation include the traditional natural ventilation, and the use of fans for mechanical forced air ventilation. Ventilation and Evaporative Cooling: Fan and Pad Ventilation is the controlled exchange of ambient air with the conditioned atmosphere within the greenhouse. The primary purpose is to reduce the air temperature, and therefore ventilation is usually controlled with an air temperature thermostat. Other environmental parameters which are directly influenced by the ventilation process are the concentration of gasses within the greenhouse air, including carbon dioxide (CO2), oxygen, and water vapor. Carbon dioxide is the most critical of the gasses because of its direct relationship to the photosynthetic processes within the green plant. An exchange of oxygen for carbon dioxide at the surface of the leaf is controlled by the diffusion process. The rate of diffusion can be drastically increased with air movement around the leaf. This benefit can be a direct result of adequate ventilation. Undesirable concentrations of gasses such as ethylene and carbon monoxide will be prevented as a result of adequate ventilation. Finally, atmospheric moisture content, typically measured as relative humidity can at times be reduced by proper ventilation, although it may be necessary to provide supplemental heating to offset excessive air cooling. The control of the ventilation system must be linked with the control of the heating system. The engineer or designer must insure that ventilation will not occur when the heating system is activated. The techniques of ventilation include the traditional natural ventilation, and the use of fans for mechanical forced air ventilation. Fan Ventilation Forced ventilation utilizes mechanical fans in combination with inlet openings (shutters or windows) for air exchange. The greenhouse air temperature can, at best, theoretically be reduced to the value of the outside temperature. Fan ventilation has been primarily utilized on plastic film covered greenhouses where ridge or side vents were not built into the design of the structure. Fan ventilation provides a positive movement of air which allows for more precise control of inside air temperature. It is has a slightly greater cooling potential than natural ventilation, primarily during the warmer periods when natural ventilation begins to be limited by low temperature differences between inside and out. Determination of the fan capacity is based on 2.4 m3 min-1 m-2 (8 ft3 min-1 ft-2) of floor area. For every square meter of greenhouse floor area, provide 2.4 m3 min-1 of fan capacity. Fans should be chosen to attain the calculated volumetric flow rate at a pressure differential of 0.25 kPa (0.1 inch of water static pressure). Particular attention to the location and the geometry of the inlet air device is crucial for effective cooling and uniform distribution. The distance from the inlet to the fan should not exceed 30 m (100 feet) for single bay or 60 m (200 feet) for multi-span greenhouses, unless a large air temperature rise (greater than 5 – 8 oC (10 - 15oF)) is tolerable between inlet and fan. The location of the inlet(s)is much more critical than the location of the fans. The cool air should enter uniformly along the wall opposite the fans and at a velocity approaching 3.5 m s-1 (700 feet per minute). A continuous vent inlet along the entire wall will insure a uniform distribution of airflow. Sufficient air speed at the inlet insures good mixing of the cold air with the inside air. At low velocities, the cold air can immediately settle to the floor, creating a cold draft which could chill plants grown on the floor or prevent the cooling of plants on benches. Inlet size is determined as 27 m2 of inlet for every 100 m3 s-1 (1.4 ft2 of inlet for every 1000 ft3 min-1) of fan capacity. An alternative type of inlet is the motorized shutter. It is typically square in shape and has only two positions, full open or closed. Shutter openings cannot distribute the air as uniformly within the greenhouse as the continuous vent window, because of their dimensions. Maintenance of the numerous shutters required in large greenhouses can make the added expense of the motorized vent window more attractive. Ventilation should occur in stages, or percentages of maximum air change capacity to prevent overshooting of the target temperature and eliminate dramatic temperature fluctuations while ventilating in the cold season. The percentage of fan capacity should be matched by a similar percentage of inlet openings for each of the ventilation stages. This will insure that the proper air velocity is maintained at the inlet for good air mixing. For example, at 1/3, 1/2 or 100% of the maximum fan capacity of the structure, the number of shutters (or the percentage of window opening) should be 1/3, 1/2 or full capacity. Cooling Systems Techniques of cooling can be organized into several categories, including pad and fan, misting, and fog. Each utilizes the evaporative cooling process to reduce air temperature, as well as fan ventilation for exchanging the moist air with dry outside air. Pad and Fan Evaporative Cooling The traditional technique for evaporative cooling is the combination of a pad and fan. The fan draws outside air through a wetted pad where it is cooled and distributed throughout the greenhouse. Although similar to the fan ventilated cooling procedure, it includes a matrix of wetted material (aspen fibers or waxed paper matrix) at the ventilation air inlet. The air passes through the matrix and it is evaporative cooled and distributed throughout the greenhouse and finally exhausted at the fan. As with all evaporative cooling systems the air temperature can be cooled below the outside air temperature. The amount of actual cooling is dependent upon the thermodynamic properties of the outside air. The theoretical minimum temperature is the wet bulb temperature of the outside air. The wet bulb depression is the difference of the dry bulb and wet bulb air temperatures, and this represents the maximum theoretical cooling that can occur within an evaporative cooling system. Air with low relative humidity (or moisture content), will have a larger wet bulb depression, and thus a greater cooling capacity, than air with high relative humidity. For conditions of high relative humidity the wet bulb temperature may nearly equal the dry bulb and greatly reduce the cooling potential. In general, cooling efficiencies for pad and fan systems range from 70 to 80%, which means that the air can be cooled to 70 or 80% of its wet bulb depression. The uniformity of air temperatures within the greenhouse between pad and fan can be poor. Similarly to the discussion on fan ventilation, as the distance from the pad to fan increases, so to does the air temperature rise. An excessive air temperature rise occurs with distances greater than 30 to 40 meters from pad to fan. To design and size the pad wall, the area of pad is determined by the airflow rate of the fan ventilation system. To minimize the pressure flow loss, an air velocity of 1.3 m s-1 (250 ft min-1) is assumed through the pad. Dividing the volumetric air flow through the pad as determined by the fan system, by the air velocity will provide the area of the pad to be installed. Water from a storage tank is pumped at a rate of 0.1 L s-1 per meter length of pad (0.5 gal min-1 per linear foot of pad). Excess water is returned to the storage tank, which should be sized to be 30 L m-2 (0.75 gal per square foot) of pad area. Theory of Evaporative Cooling The process of evaporation consists of a state change of water from a liquid to gas which requires energy known as latent heat of vaporization. This is approximately 2400 kJ kg-1 (1050 BTU lb-1) of water. Cooling occurs when sensible heat energy is absorbed from the greenhouse and converted to latent heat energy in the water vapor. In the process the atmospheric absolute humidity is increased. The psychrometric chart is utilized to represent the state points of the air/water mixture for each of the air conditioning processes. The horizontal line represents sensible heating, as the dry bulb temperature increases, while the moisture content is unchanged. The energy content of the air is determined from the change in enthalpy. For evaporative cooling, follow the wet bulb line. It represents the removal of sensible heat by the addition of moisture to the air at a constant wet bulb temperature. This is the process of evaporative cooling, which increases the absolute humidity, the relative humidity and decreases the air temperature. Natural Ventilation and Shading The rate of air movement under natural conditions is dependent upon the laws of physics describing how warm air rises. Natural convention occurs when less dense, warm air rises while the cool air of greater density settles. Natural ventilation takes advantage of this phenomenon when the air temperature within the greenhouse increases to a value which is greater than the outside ambient temperature. The warmer air rises through an opening located near to the ridge of the structure. The traditional glass covered greenhouse has been designed with both ridge and side openings (if single span or ground-to-ground design), and only at the ridge for a multi-span, gutter-connected design. This procedure has worked very well in areas where the solar radiation is not extreme, or where a cloud covered sky is common, or where natural winds are consistent. Two factors of major concern: 1) The rate of air movement is dependent upon the difference of the air temperatures between the inside and outside greenhouse locations. This has direct control implications because the greater the difference the more rapid the air exchange. The rate at which the air temperature may be changed is directly dependent on the environmental conditions of the moment. Thus the same opening size, will provide different rates of air exchange on different days, depending on the temperature difference. (2) The inside air temperature of the greenhouse can never be less than the outside air temperature, and unless the winds are very consistent and the greenhouse is oriented properly, the inside air temperature will always be greater than the outside temperature. This can become a major problem during the warm season, when the maximum cooling is required but the temperature difference is the least. In the cooler period of the year or whenever the outside air temperature is less than the desired air setpoint temperature, natural ventilation works very well. However during the warmer periods, the greenhouse will exceed the desired setpoint temperature. Within regions of high solar radiation and/or high ambient air temperatures above the desired setpoint, several design factors must be considered for optimum air exchange and ventilation. These include: S, the ratio of the area of vent opening to the ground area covered by the greenhouse. An area ratio of 0.10 to 0.20 is recommended for the m2 vent / m2 floor area. U, the wind speed [not wind direction] greater than approximately 4.5 m s-1 (14 ft s-1, 10 MPH) is also important. Together, these two parameters account for more than 50% of the ability of a structure to cool by natural ventilation. Other factors which influence natural ventilation air exchange, but are of slightly less importance are: h, the average height of the greenhouse, determined as the ratio of the volume of the greenhouse and the floor area of the greenhouse (volume m3 / floor area m2 ). H, the vertical distance between the air inlet and air outlet of the greenhouse. A greater distance increases the ‘chimney’ effect, which enhances natural ventilation. Difference of Inside Air and Outside Air Temperature, rate of air movement is increased as this difference is increased. Cr, discharge coefficient & wind effect coefficient, which is a function of the physical design of the inlet and outlet openings. In general, it is important to enhance both the wind effect and chimney effect, although there is little ‘control’ over the wind, other than to properly locate the greenhouse within a wind zone. The chimney effect can be enhanced by increasing, H, the distance from inlet to outlet. The outlet should be at or near the peak of the roof to maximize H, and to eliminate hot air trapped above the outlet opening. A continuous vent opening is preferred to smaller, intermittent openings. Shading Greenhouse shading is a procedure for cooling which attempts to reduce the amount of solar radiation which reaches the plants. By reducing the solar load on the greenhouse, the air temperature difference is smaller, making the absolute air temperatures inside the greenhouse can closer to the outside temperatures. Secondly, the leaf surface temperature can be significantly reduced. This may be the more important factor, since it is the plant processes within the leaf such as photorespiration, an unwanted process that consumes the stored energy reserves within the leaf which is affected. When leaf temperature is not a limiting factor to net photosynthesis, shading may reduce the growth and development rate of the plants by limiting photosynthesis as a result of lowered light intensity. This could prolong the plant growth time needed to reach maturity or inhibit plant quality for certain crops. Typically during the time of the year when shading is needed for the purpose of cooling, solar radiation is not a limiting growth factor. However a good control system could prevent most shade related problems. It can easily deploy an automated shade curtain only at desired times of the day. This control could be based on the measured solar radiation intensity or more simply on the time of the day. The traditional means for shading has been to apply a semi-permanent shading compound to the outer glazing of the greenhouse. Glasshouses and more recently polyethylene greenhouses have used a white liquid which could easily be applied by spraying. When dry the compound leaves a white film which reflects up to 50% of the solar radiation. After several months of weathering the compound has been nearly washed away. When frost occurs it is supposed to be easily washed off. Some cleaning may be necessary. One product is from Continental Co. called Kool-Ray which has several formulations (regular and EZ Off). Another procedure for shading consists of attaching a black polypropylene mesh screening on the outer glazing. The density of the mesh determines the amount of shading desired. Shading can be obtained from 30 to 92%. The material can be made in widths from 1.8 to 7.3 m (6 to 24 feet). Application and removal of either of these shading procedures is typically completed once for each season. They do not allow for a choice of shading on a day-to-day basis, should it be necessary. More recently, internal shading systems have been installed. They may be the same system which is utilized as the energy saving curtain in the night. These are typically attached to mechanisms for deploying and retracting the curtain after sunset and prior to sunrise. However they can be utilized during the day for shading assuming that light transmission is not severely reduced. In some instances, a separate system for shading, in addition to the energy curtain is installed. Each would be used independently. Retractable roof greenhouse designs can be utilized as shade structures. Retractable roof greenhouses can be covered with translucent, water-impermeable plastic materials, and be completely opened to the outside environment, or closed to provide a traditional plastic greenhouse structure and environment. They may also be covered with a woven, water-porous, shade curtain material, and be completely opened to the outside, or closed to provide a traditional shade greenhouse environment. The exciting feature in both of these designs is for protected plant growth, with the option for an open cover plant growth similar to outdoor production. Air Heating Design, Operation and Expectations The goal of all environmental control systems is to enhance the growth of the plant, and provide a mature crop in timely fashion, with desirable quality as demanded by the market of the producer. The process should be efficient, in order to maximize profits and require limited operational inputs of energy, labor, materials, etc. In terms of plant processes, enhanced growth should maximize photosynthesis; minimize respiration; optimize transpiration, assimilation, and carbon partitioning; and direct the products of photosynthesis to the specifically wanted plant components. These may be the vegetative parts including roots, shoots, leaves, or alternatively, the reproductive parts, such as the flower, fruit, or seed. If the plant requirements are the basis for all greenhouse systems design, then the basis for satisfactory crop production, is the determination of the proper combination of setpoint values for each environmental parameter, at each growth period, throughout the life of the plant. In general, the temperature parameter is the primary focus of most climate control systems. This may include the air temperature surrounding the aerial portion of the plant, as well as, root zone and leaf temperatures. Heating systems may utilize hot air or hot water to increase air temperature during the cool season. There are numerous systems to generate the heat, such as with direct-fired unit heaters within each greenhouse bay, or centralized hot water boilers that pump hot water to multiple greenhouse units. There are also various heat distribution systems that heat the greenhouse air (air-to-air, and water-to-air heat exchangers), the soil and plant root zone (bench and floor heating), and the leaf surface (radiant heating). The selection of a particular heating system should not only depend on cost, but also its effective integration within the crop production system and management procedure. However, the first requirement in the design of a heating system is to determine the size needed, in terms of energy requirement, or heat load of the greenhouse. Determining the design heat load of the greenhouse Calculation of the design heat load of the greenhouse is required for determining the size and type of the heater and heat distribution systems within the greenhouse. The heat load is a function of the: insulation value of the greenhouse glazing surface area of the greenhouse temperature difference between inside and outside the greenhouse radiation properties of the covering infiltration through covering The maximum greenhouse heat loss (BTU per hour) must be less than or equal to the capacity of heater. This can be determined from the following equation: Q = U x SA x (Tin,min - Tout,min) U -- overall heat transfer coefficient, (Btu hr-1 ft-2 oF-1 ) SA -- total surface area of greenhouse, (ft-2 ) Tin,min -- minimum inside air setpoint temperature (oF) Tout,min -- minimum outside air temperature (oF ) Q -- heat loss from greenhouse (or heater size) (Btu hr-1 ) The value of the overall heat transfer coefficient, U, is primarily related to greenhouse covering, and somewhat to the structural design. That is, whether the greenhouse is covered with a single or double layer, film or rigid plastic, glass or plastic, or is a single or multi-span structure. The overall heat transfer coefficient (U) combines all of the heat loss terms into one general term. It consists of conduction, convection and radiation components, as well as, air infiltration. It is furthermore, an average value for many environmental conditions. Overall Heat Transfer Coefficient U, (Btu hr-1 ft-2 oF-1 ) single layer glass 1.2 single layer P.E. (polyethylene) 1.2 double layer P.E. 0.8 double-walled, structured plastic, 0.6 PMMA or PC (acrylic, polycarbonate) The temperature difference (Tin,min - Tout,min), requires knowledge of the minimum expected outside air temperature for the winter season (Tout,min), and the desired minimum inside air setpoint temperature, or the desired/required air temperature for plants that are to be grown. Energy conservation procedures to reduce heating fuel consumption include: reducing the size of the greenhouse, selecting a covering with a smaller U-value, reduce the minimum allowable inside air temperature, provide additional energy conservation systems, or located the greenhouse in a warmer climate. The most common and typically most cost-effective energy conservation technique is the internal energy blanket system. This system can reduce heating energy consumption from 30 – 40%. This system could also be used as a summer shading system with proper selection of blanket material. In all greenhouse structure designs, a space for the energy blanket should be provided, whether the system is initially installed or not. Within a gutter-connected greenhouse, the blanket can be located at the height of the gutter. When not in use during the day, it can be tightly packed beneath the gutter to minimize shading to the plants below. Hot Air System A self-contained unit heater can be mounted directly within the greenhouse bay that burns natural gas or propane. The exhaust gases of the heater are vented through the roof or end wall of the greenhouse bay. The unit heater has capacity to heat one bay or zone of the greenhouse. It distributes heat by forced hot air that is blown into the bay by an electrically powered convection fan through a convection tube which extends the entire length of the bay. The convection tube is typically 61 cm (24 in) in diameter, and made of clear P.E., with holes spaced along its length for distribution within the bay. An alternative to the unit heater is a heat exchanger that is connected to a hot water source, such as a remote hot water boiler. It functions much in the same way as a unit heater, providing hot air within the bay with an electrically powered convection fan, but it has no combustion unit. Hot water must be pumped from the remote boiler to the heat exchanger at a specified water temperature, water flow rate and water pressure. Hot Water Pipe Heat energy from the boiler can also be delivered to the greenhouse plant growing space by an array of water pipe which carry the warm water to each bay, and distribute the heat through a series of pipe loops within the bay. The 5 cm diameter pipes are typically located overhead and above the crop, or alternatively may be located beneath the benches. The hot pipe can provide a much more uniform and more accurately controlled plant temperature, than hot air systems. The capability of a hot water heating system for distributing the heat the plants is less affected by plant row direction within the greenhouse than a hot air system. The heating pipe network is uniformly spaced throughout the entire greenhouse area, and typically distributes the heat more uniformly. The hot water pipes may be placed at the perimeter walls, overhead, at the base of the plant, or in a combination of each. Heating pipes near the base of the plant provide warmed air which rises through the plant canopy, providing a desirable plant microclimate. Bottom Heating Systems Root zone or soil heating systems can provide uniform heat beneath the crop, as with a concrete floor heating system, or a bench heating system. A network of small diameter plastic pipe is placed within the floor of the greenhouse, or on top of the benches. Warm (not hot) water at 38 – 40oC (100 - 105oF) is used to warm the plastic pipe which then heats the surrounding air or floor media (earth, sand, gravel or concrete). Heat is provided at a slow rate and at a temperature less than 27oC (80oF) at the floor or bench surface such that the crop is not damaged. A separate supplemental heat system is required to maintain the greenhouse air temperature in addition to the floor or bench system. Generally, however, air temperatures can be slightly reduced when bottom heating is used. Low temperature water such as from reject heat sources can be used in bottom heating systems. Warmed floor heated systems offer some protection from short-term heater failure due to the stored energy in the mass of the floor. Concrete floor heating systems are used extensively with ebb and flood watering systems. Radiant Heating Systems Radiant heating systems use extremely high gas temperatures created by blowing combustion gases through a metal tube which is mounted high overhead and that extends along the length of the greenhouse. The hot metal tube directly heats the plant leaf by infrared radiation without warming the greenhouse air. Natural gas is burned inside the tube, and then is exhausted outside the greenhouse. The temperature of the metal tube is not uniform along its length, and therefore it will cause non-uniform leaf temperatures within the crop. Determining actual heating costs of an operational greenhouse The procedure to determine the design load of a greenhouse heating system (i.e. "sizing" the boiler) is different from the procedure to determine the projected operational heat demand and subsequent fuel consumption during any given day (or entire heating season). The selection of the size of the boiler is primarily dependent upon the greenhouse surface area, insulation properties, and the maximum expected difference of the inside setpoint air temperature from the minimum expected outside air temperature. This calculation represents the design condition for the worst-case situation. However the real operating costs to heat the greenhouse are dependent on the difference of the actual inside and outside air temperatures at each and every moment of the heating season, and the length of time that each of those temperature conditions occurs. Thus the design heating load capacity of the boiler may rarely (or never) be reached, but the boiler must provide heat at some level, which is less than the design capacity, nearly all the time during the heating system. Unless the air temperature difference is known on an hourly basis through historical weather data or by real time data acquisition system, the Degree Day Procedure can be used to determine the amount of heat that will be needed over a given period. The Degree-Day value is calculated from an arbitrary base temperature, 65 oF. The daily average air temperature is subtracted from this base temperature to obtain the Degree-Day value for that day. Degree-Day: D.D. = (65 – “average day air temperature”) The “average day air temperature” is a simple average of the minimum and the maximum temperature of the day, [(Tmin + Tmax) /2]. This seems to be a very poor indicator of the true day temperature, but when used in the Degree-Day procedure, it can provide a reasonably good estimation of the heating required for a greenhouse. Knowing DD value for any given day (for example from a table of historical weather records) then one can calculate the average day air temperature of that day, and then can use this value in the heat loss equation. For example, if today was 40 degree-days, then the average day temperature was 25 oF (i.e. 65 – 40 = 25). If the setpoint temperature for the greenhouse is 60 oF and the U and SA values are known, then the heat, Q, needed for today was: Q = U x SA x (60 - 25) Btu hr-1 x 24 hr Knowing the heat content of the fuel and the efficiency of the boiler in use, then the number of gallons of fuel required for this day can be determined. This procedure assumed that there was no daily energy gain from solar radiation; and, that the inside to outside air temperature difference was the same for the entire 24-hour period. This procedure for energy consumptive use is usually determined on a monthly basis. For example, if the month of December has historically provided 930 Degree-Days and knowing that there are 31 days in December, then the average day temperature in December can be calculated as: (930 DD/Dec) / (31 days/Dec) = 30 DD/day therefore, the average day air temperature would be D.D. = (65 – “average day temperature”), or 30 = (65 – “average day temperature”) = 35oF therefore, the heating requirements for that month would then be Q = U x A x (60 - 35) Btu hr-1 x 24 hr x 31days Each month of the heating season can be determined separately, and added together to obtain the yearly energy costs. ____________________________________________________________________________________________ Paper # E-125933-15-02 Supported by CEAC, the Controlled Environment Agricultural Center, College of Agriculture and Life Sciences, University of Arizona. (Energy Management Heating Cooling SGHVGA.doc) |
