A Solar Powered Liquid-Desiccant Cooling System for Greenhouses

Abstract

Liquid desiccant systems are of potential interest as a means of cooling greenhouses to temperatures below those achieved by conventional means. However, only very little work has been done on this technology with previous workers focussing on the cooling of human dwellings using expensive desiccants such as lithium salts. In this study we are designing a system for greenhouse cooling based on magnesium chloride desiccant which is an abundant and non-toxic substance. Magnesium chloride is found in seawater, for example, and is a by-product from solar salt works. We have carried out a detailed experimental study of the relevant properties of magnesium rich solutions. In addition we have constructed a test rig that includes the main components of the cooling system, namely a dehumidifier and solar regenerator. The dehumidifier is a cross-flow device that consists of a structured packing made of corrugated cellulose paper sheets with different flute angles and embedded cooling tubes. The regenerator is of the open type with insulated backing and fabric covering to spread the flow of desiccant solution. Alongside these experiments we are developing a mathematical model in gPROMS® that combines and simulates the heat and mass transfer processes in these components. The model can be applied to various geographical locations. Here we report predictions for Havana (Cuba) and Manila (Philippines), where we find that average wet-bulb temperatures can be lowered by 2.2 and 3°C respectively during the month of May.

Liquid desiccant dehumidification and regeneration process to meet cooling and freshwater needs of desert greenhouses

“…The shown desiccator integrates cellulose pads with embedded heat exchange pipes. It is based on the design described by Lychnos and Davies [15,25]. The cellulose pad provides the surface area for air to desiccant contact and humidity removal.…”

Solar pond powered liquid desiccant evaporative cooling

“…Commonly used desiccants include aqueous solutions of lithium chloride, calcium chloride, lithium bromide and triethylene glycol. Other desiccants include seawater bitterns, MgCl 2 [16], KCOOH, glycols like triethylene glycol (TEG), diethylene glycol (DEG), MEG, propylene glycol, and mixtures of desiccants LiCl + LiBr or LiCl + CaCl 2 [15,17].…”

Modelling and experimental verification of a solar-powered liquid desiccant cooling system for greenhouse food production in hot climates

“…This approach applies heat and mass balance to a differential control volume at some position x from the inlet of the regenerator to formulate an ordinary differential equation, the solution to which gives an expression for the rate of evaporation per unit area of the surface. The regenerator theory is presented in detail in Appendix A and reference [51]. Fig.2 shows the detailed arrangement of the desiccator which contains several sections of porous medium (CELdek®) each inserted between bundles of cooling tubes.…”

“…Some experiments were carried out under drier conditions (RH 60-63%) but showed a significant drop of the performance and are not reported here (see ref. [51] for more details). Table 1 shows the conditions of the five experiments of this study.…”

“…is the liquid desiccant mass flow rate per unit width, des m is the salt mass flow rate per unit width, h m is the mass transfer coefficient based on pressure gradient, P 0 and P amb are the initial vapour pressure of the solution and the ambient pressure respectively, U L is the overall heat loss coefficient,in s, X is the initial liquid desiccant concentration and ps C is the specific heat capacity of the liquid desiccant.The following linear expression is used to approximate the variation of vapour pressure with temperature and concentration over small intervals: and c are calculated from experimental values[59] seeTable A.1. The detailed derivation of the above analytical solution can be found in ref [51]…”