Potential use of phase change materials in greenhouses heating: comparison with a traditional system

In order to use solar radiation as thermal energy source, heat storage equipments result necessary in each application where continuous supply is required, because of the natural unsteady intensity of radiation during the day. Thermal solar collectors are especially suitable for low temperature applications, since their efficiency decreases when an high inlet temperature of fluid flowing through them is established. On the other hand, low temperatures and low temperature gaps, above all, make very difficult to use traditional sensible heat storing units (water tanks), because of the very large amounts of material required. In this work, a traditional sensible heat storage system is compared with a latent heat storing unit based on phase change materials (PCMs). As a case study, a 840 m3 greenhouse heating application was considered with an inside constant temperature of 18°C. It is thought to be heated by using single layer plate thermal solar collectors as energy source. Inlet temperature of the collectors fluid (HTF) was fixed at 35°C (little higher than melting temperature of PCMs) and a constant flux of 12 l/m2•hour was established as technical usual value. At these conditions, 215m2 solar panels exposed surface resulted necessary. The sensible heat storage system considered here is a traditional water tank storing unit equipped with two pipe coils, respectively for heat exchanges with HTF from collectors and water flux for greenhouse heating. Available T for heat exchange is estimated as the difference of minimum HTF temperature (in outlet from the collectors) and the required water temperature for greenhouse heating. The latent heat storing unit is instead a series of copper rectangular plate shells which a phase change material is filled in (Na2SO4∙10H2O). Heat transfer fluids flow through thin channels between adjacent plates, so that a large heat exchange available surface is achieved. The developed computational model (Labview software) permits to superimpose heat exchanges daily curves between heat storing materials and heat transport fluids (for both of the fluids and the heat storing equipments) on the energy supply/demand ones, respectively calculated on the basis of greenhouse energy demand and solar collectors dimensions, characteristics and efficiency. In this manner, units design is achieved by changing thermal energy storing units dimensions, in order that the corresponding heat exchange curves coincide with the previously calculated ones. Successively, among all the possible configurations, the ones showing lower units volumes and less amount of storing materials are chosen as the optimal design solutions. It has been proven that PCMs materials are much more suitable for low temperature applications than sensible heat storing materials (water). In the case of water tank, an about 15,8m3 total volume is required while for PCMs equipment the total volume of storing unit is reduced to about 2,2 m3, such as about seven times total volume less. Besides, according to the simplified and steady state model calculations, PCMs unit shows a better response to the hourly energy fluctuations of solar collectors and greenhouse demand than water tank unit. This is especially due to the high available exchange surface achieved in proposed arrangement.