Heat Transfer And Energy Conservation Research On The Phase Change Heat Storage Technology In Greenhouse

The glass or plastic greenhouse, which is also known as solar greenhouse, is designed for using the solar energy to increase the indoor air temperature and satisfing the temperature requirements of the plants for growing, which provides an appropriate small climatic environment for the plant growth, achieves diversity, anti-season, anti-zone in the plant production in any season, and makes full use of the northern soil, and so on. However, due to the intense solar radiation in the daytime, the indoor air temperature is usually too high in the daytime, resulting that ventilation must be needed to set free the redundant heat, which wastes a lot of solar energy to a certain level. At the same time, because the poor heat insulation and the ineffective thermal storage characteristics of the traditional greenhouse, heating at night in winter is needed in order to create an environment meeting the temperature demand of plant growth.In many areas of north China, the solar energy resource is abundant in winter with the properties as long-time sunshine and intense solar radiation. however, the solar energy isn't always exploited properly in the greenhouse.Basing on the building characteristic of the glass or plastic greenhouse itself and the passive heat storage and release property of the phase change material, this paper introduces a new concept of passive solar greenhouse with the walls of heat-storage style, which is built by directly applying the new kind of composite phase change heat-storage material developed by our research group to the inner surface of walls in common greenhouse. The PCM wallboard, owing to the PCM's characteristic of absorbing or releasing large amount of latent heat isothermally, can store heat in the daytime. It does this when the surface temperature of the PCM wallboard is higher than the phase change temperature of the PCM in the daytime, thus cutting the peak temperature, and releasing heat while the surface temperature of the PCM falls under the phase change temperature of the PCM at night, thus completely or partially instead of normal energy sources, enhancing the indoor air temperature, reducing the heating energy consumption, and achieving the aim of energy conservation to a certain extent and " time shifting " in the use of solar energy.Grounding on the energy conservation concept above, this paper primarily introduces the experimental study. A small size experimental greenhouse (including the PCM greenhouse and the ordinary greenhouse) has been built, and repeated and long contrastive experiments have been completed. Experimental results show that, when the outdoor air temperature is high and the solar radiation is intense in the daytime, the PCM greenhouse presents a good energy conservation characteristic without heating by the electricity. And when the outdoor air temperature is lower and heating by the electricity is needed, the electricity consumption of the PCM greenhouse can be lower than that of the ordinary greenhouse, and the power saving rateηreaches 6.3% as a result,so long as the indoor air temperature in the PCM greenhouse at night is 2～3℃higher than the phase change temperature (in the experimental stage I ), which better reflects the energy saving in the greenhouse with the new composite phase change heat-storage material.On the basis of primary experiments, the mathematical control heat transfer equations of ordinary greenhouse and PCM greenhouse are established. Through establishment of phase change heat transfer model of the greenhouse, simulations and calculations according to the heat storing/releasing characteristics of the PCM are made by effective heat capacity method and the software MATLAB. In virtue of the effective heat capacity model, the heat transfer problem of the PCM, which is described by three equations (i.e. solid state, liquid state and solid-liquid coexistence state), can be transformed into "single- phase" non-linear conduction problem in the whole calculation region. With the software MATLAB7.0, firstly, programmes for ordinary greenhouse is established and operated. On the basis of results of ordinary greenhouse, the programme for PCM greenhouse is established which is obtained by optimizing, amending and perfecting the programme of ordinary greenhouse. By the results of PCM greenhouse, we discuss the effect of thermal parameters of the phase change material, (such as the phase change temperature, the phase change enthalpy, the density, the thermal conductivity, the heat capacity and so on) on the energy saving rate of heating season comparing with the ordinary greenhouse. Comprehensive study shows that the energy saving rate of heating seasonηcan get to 2.4% or higher when the optimal thickness is set at 30 mm, the phase change enthalpy of the PCM is set at 120kJ/kg, the density is set at 1200 kg/m~3, the thermal conductivity is set at 0.6 W/(mK) and the phase change temperature is set at 20℃.At last, simulational computation for PCM greenhouses in different regions such as Beijing, Lasa, Shanghai, Herbin and Wulumuqi which are feature as different weather conditions are operated. The influence rules of the PCM greenhouse on the coal consumption and the inner surface temperature of the north wall have been discussed. With the computational results the feasibility and energy conservation characteristic of the PCM greenhouse using in different regions have been estimated. Through favorable combination of the construction characteristic of passive solar greenhouse and the PCM's characteristic of absorbing or releasing large amount of latent heat isothermally, this paper will promote the use of solar energy in the development of new space, open up a feasible way for the new socialist rural economic construction and energy conservation in passive solar greenhouse, and provide valuable references for the application of PCM heat storage wallboard in the country greenhouses or ordinary residential buildings.