David R. Mears

Evaluating Energy Savings Strategies Using Heat Pumps and Energy Storage for Greenhouses

As energy costs are increasing, many greenhouse operators are re-evaluating energy consumption and savings strategies. In most cases, updating older heating systems to more efficient units in addition to the use of double layer glazing, insulation materials, and energy curtains significantly reduces fuel consumption. Insulating greenhouses must not conflict with the need for high light transmission through the structure. Since solar radiation loads often significantly exceed the instantaneous heat requirement of a greenhouse, many ideas have been proposed to capture this excess heat and store it for later greenhouse heating. Heat pumps are promising for use in an integrated cooling and heating system. In the study described in this paper, a simple spreadsheet approach was used to evaluate the performance of a system utilizing a heat pump and water storage. The evaluation bases its calculations on historic hourly weather data to determine hourly cooling and heating rates and storage status. The calculations allow for evaluations of the appropriate size of the heat pump, storage device, and heat exchangers. The calculations are used to investigate storage capacities that are sized for one to a few days harvest of surplus heat from the greenhouse for a range of percentages of peak cooling requirement. The model includes the option of utilizing a geothermal source for the heat pump to charge the storage during periods when greenhouse cooling is not required. The first study presented examines the impact of increasing thermal storage capacity on heat utilization from a generic co-generation system. The second considers a specific, natural gas fired, fuel cell system for various sizes of greenhouse at two different locations and includes the utilization of CO2 from the reformer section. The heat pump study looks at the relationships between capacities of the heat pump and storage for two different locations. Provided hourly weather data are available for other sites, the spreadsheet approach can be used for other locations across the world.

Open-roof Greenhouse Design with Heated Ebb and Flood Floor

An open-roof greenhouse production system with a heated ebb and flood floor irrigation system is being developed and evaluated. In addition to continuous roof vents, the greenhouse is equipped with sidewall vents to allow for ventilation during windy and rainy conditions. This paper discusses the design details as well as the instrumentation used for the evaluation. Preliminary data, collected over a 2.5-month period, of light and temperature conditions are presented. The greenhouse temperature closely followed outside temperature conditions for the entire measurement period when the inside temperature exceeded the set point. The inside light conditions were significantly affected by the greenhouse structure, and inside light intensities around solar noon could exceed outside light intensities due to reflection from the opened roof segments.

Numerical Modeling of Greenhouse Floor Heating

A numerical simulation model of a greenhouse floor heating system was developed and validated using data collected in a research greenhouse located at Cook College, Rutgers University, New Brunswick, New Jersey. The model was then modified and used to evaluate two different heat pipe diameters and spacings that are typical in the greenhouse industry today: 13 mm (0.5 in.) diameter pipe placed on 22.9 cm (9 in.) centers, and 19 mm (0.75 in.) diameter pipe placed on 30.5 cm (12 in.) centers. Two heat pipe elevations within the solid concrete floor system were also simulated, and the effects of the pipes vertical position, diameter, and spacing on surface heat flux, surface temperature, and surface temperature uniformity were evaluated. The simulation results showed that the smaller diameter pipe placed closer together and at a lower elevation provided the best temperature uniformity without compromising other performance criteria. The model was then further modified to simulate flats with growing media placed on the floor surface. Model simulations were conducted for six different supply water temperatures ranging from 32.2°C (90°F) to 60°C (140°F), while maintaining a target ambient greenhouse air temperature of 15.6°C (60°F). The simulation outputs showed that using the smaller diameter pipe placed closer together resulted in a higher surface heat flux, a higher growing media temperature, and greater temperature uniformity within the growing media, for each supply water temperature simulated.

Design Considerations for Small-Scale Pipe Greenhouses to Prevent Arch Buckling Under Snow Load

Small-scale single-span pipe frame greenhouses (pipe houses) are widely used in Japan because these lightweight structures are suitable for construction by the farmers thus reducing labor cost. However, farmers without benefit of any structural analyses have erected these pipe houses. As a consequence, these pipe house structures are very weak and frequently damaged by buckling under snow load. To improve the design, stress and buckling analyses were conducted. The result shows that an unmodified design pipe house has a very low limit load (192 N/m2) with collapse caused by bending moment. In contrast, a pipe house reinforced by two steel wires, has higher limit loads. Both maximum stress and critical buckling analyses are necessary. To verify the numerical structural models, loading tests were also conducted. Pipe house designs reinforced by two steel wires showed 1.6 to 2 times the limit load as the unmodified structure. There was a good linear relationship between limit loads from FEM analysis and from loading test.

Computer-aided desing of a greenhouse waste heat utilization system

Abstract The objective was to design a prototype greenhouse heating system utilizing warm water from a specific electric power generating station. To take into account the effects of changes in factors affecting greenhouse energy requirements, computer simulation was extensively used to evaluate a number of possible designs. Site-specific information was available on the weather and the temperature of the power plants condenser discharge water at hourly intervals over the entire year. The computer simulation program was used to compare different design options and to explore the interactions between heating systems, power plant water and outside ambient temperatures, and temperature control strategies. Using the results of the computer studies a 1.1-ha prototype was designed and constructed. It operated successfully throughout the 1980–81 heating season. In the fall a poinsettia crop was produced and in the spring there were lilies, pot mums, other potted plants and a bedding plant crop of tomatoes.

Thermal condition in a compact sunroom for fresh vegetables production

Thermal conditions in a 3 m2 sunroom attached to a residence building were estimated from a vegetable production perspective. The set points of the air temperature were 12–24 °C, an appropriate temperature range for most vegetables. The air temperatures in the full scale model sunroom could be maintained over 12 °C with up to 417 W supplemental heating for the measuring period. In the sunroom, the averaged overall heat loss coefficients between the sunroom and the outside and between the sunroom and the residence building for 24 days during nighttime in February and March were 2.6 and 13.0 W/m2 °C, respectively. The tendency of the diurnal range of air temperature in the sunroom could be roughly simulated for the measuring period except in some daytime cases in February, although the simulation method was a simple and approximate energy balance model with the averaged overall heat loss coefficients.

Comparing Greenhouse Floor Heating Designs Using CFD

Floor heating in greenhouses has become more and more popular over the past decades because of the considerable benefits warm floors can have on greenhouse crops. In addition, bottom watering is beneficial for many crops. Making use of the synergy of both bottom heat and bottom watering, many greenhouse growers are utilizing heated ebb and flood floors as their main plant productions system. While typical warm floor systems work well, they may not be fully optimized. Accurate and flexible computer models can be extremely valuable design tools when applied to the study of greenhouse environmental control systems and can answer many questions without the time and expense associated with experimental research. A model was developed and validated, using computational fluid dynamics (CFD) software, of a typically designed warm floor system. The model was then modified to investigate the effect of heating pipe diameter and spacing, vertical position in the floor slab, and soil thermal conductivity on heat flux through the floor and temperature uniformity at the floor surface for typically designed commercial greenhouse floor heating systems. Thirty two simulations were completed to compare the performance of two different pipe diameter/spacings, two pipe elevations in the floor slab, and two soil conductivity values, each with four pipe water and greenhouse air temperature combinations. The results showed that soil thermal conductivity had little effect on temperature, heat flux, or temperature uniformity on the surface of the floor. Raising the pipe position increased both the floor surface temperature and surface heat flux, but reduced the surface temperature uniformity and had little impact on reducing soil heat flux. Using a smaller diameter pipe with a closer spacing increased the temperature, heat flux, and temperature uniformity on the surface of the floor without increasing the percentage of the total heat input to the floor that was lost to the soil below the floor.

Greenhouse Floor Heating

Making use of the synergy of both bottom heat and bottom watering, many greenhouse growers are utilizing heated ebb and flood floors as their main plant productions system. While the typical design and control strategies that are implemented for these systems work well, they still may not be optimized for energy efficiency or crop benefit. Accurate and flexible computer models can be extremely valuable design tools when applied to the study of greenhouse environmental control systems and can answer many questions without the time and expense associated with experimental research. This paper describes the first attempt to develop and verify a model of a floor heating system that is installed in a research greenhouse located at Cook College, Rutgers University in New Brunswick, NJ. This model considers the simple case of a heated ebb and flood floor without a crop, and will be used to develop more complex models. The model’s output under predicted the average floor surface temperature measured in the greenhouse by an average of 3.20oC (5.76oF), for eight combinations of pipe water temperature and greenhouse air temperature considered, with a standard deviation of 0.28oC (0.49oF). By raising the external radiation temperature used as input to the model by an average of 8.63oC (15.53oF), the models predicted average surface temperature matched the measured average surface temperatures for all eight cases considered. In order to confidently verify the model’s output, a better method for determining the external radiation environment is necessary. More complete models need to be developed that include the soil below, the crop above, and all the thermal relationships that exist between them and the greenhouse. With such models, the thermal performance of these systems can be better understood, and the effects of changing design parameters as well as control strategies can be determined.

Greenhouse Environmental Control for Indian Conditions

It has been noted by several speakers at the seminar that there are very few functioning greenhouses in India with the exception of a few at various research centers. This is also the impression the author has developed during extensive travels to a number of research centers and agricultural production areas within India over the past several years. It has also been observed and noted that in many research centers the greenhouse facilities are not actively utilized. In some cases this is due to environmental control equipment being out of order and in others the problem is that the installed equipment is not adequate even if functioning properly. The most commonly observed problems are related to inadequacies of repair or design of cooling equipment.