In conventional greenhouse hybrid heating systems, a non-oil heating device such as a heat pump or a wood-pellet heater is used in combination with an oil heater. The system needs to be designed not only to minimize the installation cost of the expensive non-oil device but also to efficiently reduce the use of fossil oils. Herein, we propose an approach to estimate the percentage of seasonal heat supplied by a non-oil heating device. By assuming that heat loss from a greenhouse is proportional to the difference between the inside and outside air temperatures, we determined the ratios of the heating capacity of a non-oil and an oil device to achieve maximum heating load. We also determined the percentages of total seasonal heat supplied by the two devices, assuming that the total heat consumption is proportional to heating degree hours. The percentage of the heat supply calculated using hourly temperature records at 11 locations in Japan was linearly related to the mean difference between inside and outside temperatures, and this relationship was consistent across the selected locations. Using these results, we derived equations that enable the estimation of the percentage of seasonal heat supplied from a non-oil heating device.
