Evaluating the economic feasibility of thermal screens in New Zealand using a mathematical model : a thesis presented in partial fulfilment of requirements for the degree of Master of Horticultural Science in Horticultural Engineering at Massey University
A mathematical model of the greenhouse environment was developed to ascertain the annual savings in heating expenditure achieved by thermal screens. Thirteen materials with thermal screening potential were investigated. Each material was modelled within glass, Agphane, and twin skin Agphane covered greenhouses, 300m2 and 1000m2 in floor area, heated with diesel, coal, electricity, natural gas, or L.P.G., to set points of 15°C and 20°C, in Auckland and Christchurch. The model consisted of two phases. Phase 1 was a steady state model of the greenhouse environment based on a series of energy and mass balances. The temperatures within the greenhouse and the quantity of heat required to hold the house at a specified set point were predicted by solving these balances simultaneously. This process enabled the average U-value for each greenhouse to be estimated. In Phase 2 of the model the annual heat load for combinations of each house size and type, cover, screen, set point, and location were estimated using average U-values from Phase 1 and meterological data indicative of Auckland and Christchurch. Using current fuel prices, annual heat loads were converted into annual heating expenditures. Using annual heating expenditure, screen life expectancy, and screen installation cost an economic analysis was conducted using internal rate of return as a measure of thermal screen feasibility. In terms of savings in heating expenditure, Black Polythene, Infrane, and Clear Polythene recorded the highest internal rate of return. It was decided that before a formal recommendation could be made further research was required to evaluate screens as summer shading or photoperiod control devices and to consider the practical problems associated with some of the screens. It was shown that returns from thermal screening were greater in Christchurch than Auckland, greater at a 20°C set point than at a 15°C set point, greater for a 1000m2 house than a 300m2 house, greatest with diesel heating in Auckland, and greatest with diesel and L.P.G. heating in Christchurch.