An evaluation of the economic benefits of active cooling and carbon dioxide enrichment of greenhouse cucumbers (Cucumis sativus L.) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Horticultural Science at Massey University

Abstract

Cooling a greenhouse with a refrigeration system rather than conventional ventilation makes it possible to maximise the fractional enrichment time for carbon dioxide, and more importantly enrich during periods of high photosynthetically active radiation. Using conventional climate control methods, enrichment is limited to periods when the greenhouse is not being ventilated, thus reducing the potential enrichment time of the crop. The objective of this study was to develop a simulation model of a greenhouse crop growing with a closed cycle climate control system, using a heat pump, with a reversible (dual) cycle, for heating and cooling. A computer implemented mathematical model developed by Wells (1992) was modified to simulate cucumber crop growth in a greenhouse of commercial size and allowing certain parameters to be set. These parameters included: two types of control system, four levels of enrichment, three crop periods, and at two locations, Auckland and Christchurch. The three crop periods chosen were 26 Jan to 26 April, 25 May to 23 August, and 20 September to 19 December. The two types of control involved conventional fan ventialtion and electric heating, and closed cycle climate control using a reverse cycle heat pump. Greenhouse carbon dioxide enrichment levels used were 350, 600, 900, 1200 μ1.1-1

. The two locations chosen were Auckland and Christchurch. An economic analysis of the results was carried out calculating Annual Marginal Return (AMR) and Internal Rate of Return (IRR) for treatments compared to control. It was concluded that carbon dioxide enrichment combined with conventional control is a worthwhile investment in Christchurch but less so in Auckland. Due to the high capital cost, carbon dioxide enrichment combined with closed cycle climate control is a less attractive investment. However, as considerable energy savings are possible with closed cycle climate control, it is worthwhile investigating other less expensive forms of closed cycle climate control. The economic feasibility of the application of this technology to other, higher value, crops is worthwhile investigating.