Energy in Agriculture最新文献

The benefits of using solar tracking mechanisms, particularly where photovoltaic arrays are used to power a water pumping system, are discussed.

The principles of designing and constructing a simple automatic solar tracking mechanism are shown. A single-axis tracking system is used, involving the rotation of the solar receiver at the rate of 15 deg per hour around a north-south axis. A water clock device, with a system of valves controlling the flow of water and so the tracking and reversing rotation mode, is used to track the sun. One hundred and forty to 320 litres of water were used in each cycle.

The results of the tests shown that the maximum error of tracking was 15 deg during a 4-week period. After that period, resetting of the mechanism was necessary.

The total cost of the materials used for construction of the tracking mechanism was approximately US$50, excluding the frame supporting the photovoltaic array.

Consumption of energy was analysed on a kori-tofu plant which processed 6 t of soybeans per day, involving 13 unit operations such as wet-milling, extraction-denaturation of soy protein, coagulation, compression, thawing, and waste-water treatment. Total energy consumption per kg of the dry kori-tofu was 35.4–27.1 MJ of fuel energy and 8.3 MJ of electrical energy. The extraction-denaturation of soy protein consumed twice as much thermal energy as the drying. The waste-water treatment consumed more electrical energy than the whole manufacturing process including the freezing and aging.

Next the reduction of the thermal energy consumption in the extraction-denaturation of soy protein was attempted. The point was to reduce the amount of the extraction water that must be heated to at least 90°C for denaturation of the soy protein. Decrease in the yield of extracted protein was overcome by using multi-extraction. The coagulation was not affected if the ratio of bound calcium to soy protein was controlled at a certain level. The consolidation became easier but its operation had to be adjusted to obtain the same quality of the consolidated cake. The series of investigations suggested that the thermal energy consumption in the extraction-denaturation of soy protein could be reduced to less than one half of the current energy consumption by only improving the extraction-denaturation of soy protein. Cost for a partially optimized sub-system is also discussed.

System performance, crop yield and cost are the major criteria for design optimization of a greenhouse solar heating system. While experimental study and evaluation of each plausible design are cost-prohibitive, resorting to detailed computer simulations would enable the prediction of system performance at specified locations.

This paper discusses the development of a design procedure for greenhouse space heating. A simulation model that describes the greenhouse thermal environment and thermal storage, is verified by short-term actual data. Monthly average meteorological data were then used as inputs to the computer program for predicting long-term system performance, as indicated by the fraction of annual heating load supplied by solar energy. Results from simulation runs suggested that monthly solar heating fraction may be correlated with a dimensionless variable that involved mean daily solar radiation on an outside horizontal surface, total capture factor of the greenhouse cover, and night-time heat load. Such correlation was presented as a preliminary guidline for designers. Crop yield and economic analyses have to be carried out before selecting an optimum design for a particular location.

One critical factor for crop energy conversion for plant growth is photosynthetically active radiation (PAR) received by the plant. While it is important to know their total solar radiation transmission characteristics in the design of greenhouse for thermal environment management, it is also essential to understand their PAR transmission capability, especially over the winter period for high-latitude regions. This paper presents the results of PAR transmission of four different greenhouse glazings, measured at both the glazing and crop canopy levels. The glazings studied were single glass, double glass, twin-walled acrylic and air-inflated double polyethylene. The first three materials were tested at a commercial rose greenhouse range (gable type) in Connecticut and the double polyethylene greenhouse (bow type) was located at Cook College, Rutgers University. Also reported is the comparison between total solar radiation transmission and PAR transmission in the double polyethylene greenhouse.

The glazing level PAR transmission showed mainly the effects of glazing materials, sky clearness and solar angle of incidence, whereas PAR received at the canopy level was strongly influenced by the greenhouse geometric configuration and internal structures. It was found that air-inflated double polyethylene transmitted a higher percentage when measured in the total solar radiation range than in the PAR range.

Utilization of warm geothermal water (40°–60°C) supplied from deep wells for heating of winter crops is being pursued in the Arava region of Israel. Systems for soil warming have been developed and tested. This paper describes the design procedure of a space heating system utilizing warm water. Polyethylene irrigation piping on the crop bed is used for heat exchange. The design, based on conventional heat transfer formulae and computer simulation, resulted in a series of curves that facilitate future practical designs. An example, utilized in commercial plots, demonstrates the calculation procedure with respect to the special problems of medium salinity geothermal water used for irrigation.