RESEARCH AND COMPARATIVE ANALYSIS OF THE SYSTEMS OF HEATING, COOLING AND VENTILATION IN GREENHOUSES

Mukatay N., Atyhanov A.K., Rozi Amut., Ospanov A.T.

Kazakh National Agrarian University, Almaty, Kazakhstan

6D080600-Agricultural Engineering and Technology

nureke_phd@mail.ru Tel:8702-166-3-99

The conditions in the greenhouse, contributing to crop yield, called the greenhouse microclimate. They can be defined as those climate variables to which the vegetation in the greenhouse and be exposed include: temperature, radiation, carbon dioxide levels, humidity, and wind speed. Energy and mass exchange between the outside air and the greenhouse effect on its microclimate. These exchanges can be described with the energy of the greenhouse and the amount of mass. The energy budget - the balance between the inner and outer flow of energy in the greenhouse, while the mass budget assumes input and outflow of the mass (water vapor and carbon dioxide). These budgets are influenced by the structure and design of the greenhouse, the environment, the state of the plant and management decisions.

1.1 Greenhouse heating systems

Greenhouses are designed to provide control of solar radiation, temperature, humidity, and CO2 levels in the aerial environment. Large energy costs are incurred to maintain the required thermal and radiant environments in greenhouses during the winter reason. During the winter season, supplemental heat at night is needed. It may be needed during the day as well. Many heating and air circulation systems are commercially available today. These systems include central heating, spaced unit heaters, perforated tube air distribution, horizontal air circulation, floor heating, etc.

Central heating systems include a standard black pipe containing hot water or steam, finned pipe containing hot water or steam, hot air distributed in sheet metal or plastic tiling, and electrical resistance strips. The piping may be placed along the outside walls, below benches or directly above plant bed surfaces for root zone heating. Naturally induced air circulation usually provides uniform temperatures within the plant canopy. Sometimes, air circulation fans may be desirable to improve the naturally induced circulation patterns.

However, reports that overhead circulating fans which pull air upward and discharge it radially outward do not effectively improve the inside air temperature uniformity in greenhouses with sidewall pipe heating systems [1].

Overhead-perforated plastic tubes with unit heaters are also used for greenhouse heating. In these systems, the discharge holes are arranged in such a way that air is not blown directly onto plants. Several other discharge units are available with or without additional circulation fans to obtain better temperature distribution within the canopy. The unit employed may differ depending on the architectural characteristics of the plants grown in the greenhouse.

Another common practice is floor heating if plant containers can be set directly on the floor. Reported that this system can provide up to 25% of the heat requirement for a double glazed greenhouse structure. It was also reported in a number of studies that soil, floor, and bench-top tainting systems for. Greenhouses conserve energy compared to more conventional natural or forced convection heated air systems. This conclusion was justified due to the relatively smaller interior space, which needs to be heated. Observed that energy consumption for the bench system was about half of the perimeter system for equal canopy temperatures.

Another heating method available today is radiant infrared heating which provides adequate leaf temperatures while permitting the use of lower air temperature set points. Several studies compared infrared radiant heating systems with hot water systems in terms of the energy consumption. Reported a net energy consumption reduction of 12% and 6% for two different configurations of infrared systems compared to hot water systems.

Compared three gas fired greenhouse-heating systems (infrared emitters, condensing boilers, and unit heaters) in cold climates. They reported that each system produced relatively uniform air temperature distributions both horizontally and vertically. However, the infrared emitters permitted equivalent leaf temperatures with lower air temperature set points (at least 3°C), wife condensing boilers providing the best energetic performance because of their high steady-state efficiency and better sizing with respect to the heating load. They concluded that high efficiency infrared emitters are needed in order to benefit from infrared heating.

Alternative energy sources such as solar energy are receiving greater attention for heating, cooling and power-generation applications. In addition, the limited supply of natural gas and the uncertainty about the oil supply in recent years has advanced the use of heat pumps for heating applications. To improve the heat pump COP and replace the fossil fuel energy source. The idea of combining the heat pump and solar energy in beneficial ways has been proposed and developed in several previous studies.

The principal advantage of using solar radiation as a heat pump energy source is that, it provides heat at a higher temperature level than other sources, resulting in an increase in COP. Typical solar source heat pumps can be operated in two basic configurations- direct and indirect. In a direct or conventional system, the refrigerant passes through the evaporator tubes in a solar collector (usually a flat plate type), where it is evaporated by incident solar energy. The heat pump extracts energy and delivers it to the load at the required temperature. In this system, the collector and evaporator functions are combined into one unit. J Shah studied the technical feasibility of using a heat pump in conjunction with a solar pond for space heating.

Krause investigated, using a. simple computer model, a space heating system consisting of an. unglazed collector, a heat pump, and a medium-term phase change ,heat storage in place of an auxiliary heating unit.

The investigator concluded that the system appears technically feasible, but that the economic feasibility depended upon future price developments. Andrews et al. (1998) presented a study which examined a series of systems that used low cost collectors and concluded that they could be cost effective when both heating and cooling were considered. Karmen (1996) compared the performance of several systems. Most of these studies demonstrated that substantial energy savings can be realized using solar heat pump systems.

However, the economics were generally marginal when initial costs and operating costs were examined.

In another study, Spieser and Pitblado determined the performance and economics of using water-to-air heat pumps to provide environmental temperature control in eight different insect growth rooms. They concluded that the heat pump system worked very well and maintained the room air temperatures within 1 °C.

1.2 Greenhouse cooling systems

During hot weather, if quality crops are to be grown, it is essential to provide greenhouses with appropriate cooling. Either evaporative cooling, refrigerated air conditioning, or ventilation may provide cooling systems, which draw outside air through underground pipes, or rock-bed units that are cooled at night.

Evaporative cooling, using current technology and available equipment, can be an energy efficient and cost-effective method of cooling. It cools air by evaporation of water.

When water evaporates into the air being cooled, it creates direct evaporative cooling,, which is the oldest and the most common form. However, when the evaporation occurs separately and the air is cooled without humidity gain, the process is called indirect evaporative cooling.

There are a number of techniques to cool the air using direct evaporative cooling. These include 1-) evaporative coolers, 2-) spray-filled and wetted surface air washers, 3-) sprayed coil units, and 4-) humidifiers.

Some evaporative cooling methods can provide economical greenhouse cooling. However, direct evaporative cooling adds moisture to the air that may increase disease problems. It was also reported by Montero that higher humidities appear to reduce leaf transpiration. The amount of useful evaporative cooling obtained depends largely on the wet bulb temperature of the air entering the equipment. High ambient relative humidities severely limit the effectiveness of evaporative cooling and misting systems because of the reduced vapor pressure differential.

Montero studied the influence of evaporative cooling systems on greenhouse environment. Compared the efficiencies of the evaporative pad system, the sonic humidifier, and the electric spanner humidifier.

They observed that the humidifiers were more effective than a horizontal pad in achieving a high evaporative efficiency [2]. In another study, Sepal developed an economic model to predict the feasibility of using rock beds to provide closed-loop cooling, and thus increased CO2 enrichment, for tomato and cucumber \operations in greenhouses. They reported that initial, results showed little promise for tomato operations; however, cucumber operations showed some potential for profitability, especially for areas with high solar radiation.

1.3 Greenhouse ventilation systems

Ventilation is a necessary component of conventional greenhouse systems. It is an important tool in "maintaining both an acceptable temperature and a relative humidity range for plants to grow trouble free. Ventilation removes the excess heat and moisture from the greenhouse. It is also a vital component in terms of introducing the outside air which has a higher CO2 content compared to the inside air, if CO2 enrichment is not being practiced. As will be discussed in Section 2.7, in energy conserving greenhouses, it is desirable to reduce ventilation rates to reduce the heat loss in the ventilation air. Some other greenhouses may have entirely closed-systems, which require no ventilation.

Greenhouse ventilation systems can be classified into two systems which are natural ventilation systems and mechanical ventilation systems. A pressure difference must be created by wind or temperature for natural ventilation to occur. To take advantage of wind-created pressure differences, it is recommended that vent openings be present on both sides and on the" ridge of the greenhouse. Greenhouses with only side vents depend on wind pressure to force air exchange and are usually ineffective.

The other ventilation system is the mechanical or fan ventilation system. The fans are usually located on the downwind side or on the end of the greenhouses, and incoming air is introduced into the greenhouse so that it mixes and is warmed by interior air before* contacting the plants. Standards recommends mechanical ventilation rates of 0.005-0.01 m³/(s.m²) for minimum winter air exchange and 0.06 m³/(s.m²) for summer cooling.

Literature:

1. Akhter, M. P., G. E. Meyer and J. A. De Shazerv. 1988. -Evaluation of passively heated and cooled Alpine greenhouse-simulation studies. Unpublished. University of Nebraska-Lincoln, Nebraska, U.S.A.

2. Albright, L. D., R. G. Reins and S. E. Anderson. 1578. Experimental results of solar heating a Brace Institute style greenhouse. Conference on Solar Energy for Heating Greenhouse and Greenhouse – Residential Combinations, Fort Collins, CO, U.S.A.