Solar Energy System Design for a Net-Zero Energy Buildings Technology

This paper combines a review of solar energy technologies, the physics of solar radiation and Sun–Earth geometry, and the design of a solar energy system intended to bring an elementary school in Gharyan, Libya, to net-zero energy operation, and it compares the site with Dayton, Ohio, where an earlier school design that is the main precedent for the Gharyan system was developed. The design couples 181 evacuated-tube collectors and a lithium bromide–water absorption chiller, sized with the f-chart, utilizability and phi-f chart methods for an annual solar fraction of 56%, with a 228 kW grid-connected photovoltaic (PV) array of 578 monocrystalline modules sized with PVWatts. The governing radiation equations are applied to both latitudes, and a monthly tilted-surface model, which reproduces NREL tilt data for Dayton within 2.1%, is used to compare the sites. Gharyan receives 48% more global horizontal irradiation than Dayton (2129 against 1438 kWh/m2 per year), has one third of its heating degree-days and 2.5 times its cooling degree-days, and the same array would yield about 1630 kWh/kWp against 1140 kWh/kWp. The design methods transfer from Dayton to Gharyan, but the collector numbers, solar fractions and PV output do not: about 135 kW of PV would meet the Gharyan load, and the electricity tariff, under 1 US cent/kWh in Libya against 11.3 cents in Ohio, explains why a payback of about five years in Ohio cannot be expected in Libya.