Carlos J. Colon — Florida Solar Energy Center, University of Central Florida
Tim Merrigan — National Renewable Energy Laboratory
U.S. Department of Energy, Building Technologies Program
Goal: Cut the delivered, life-cycle energy cost of solar water heating systems in half by 2005.
Timeline: 1998 — "New Concepts for Solar Systems" RFP; 1999 — "Low-Cost Solar Systems" RFP to industry, Phase 2 of New Concepts; 2000 — Select best concepts and focus R&D.
A thermal absorber installed in contact with the roof sheathing, underneath the weatherable roof surface and not exposed directly to solar radiation. (An "invisible" collector.)
- Flexible Roof Facility (FRF) at Florida Solar Energy Center
- RISA build-up: 1/2" PEX-AL-PEX with Al heat transfer plates; 3/4" rigid insulation
- Direct circulation system: collector, 80 gallon storage tank, 1.2 gpm circulating pump, differential controller; freeze prevention and pressure relief
- RISA Phase 2: Thermal performance testing of two prototypes — asphalt shingle roof and metal roof
- Shingle RISA: Low temp (35°C inlet): 5,788 Btu/day, 208 Btu/ft²/day, ~1.7 kWh/m²/day; Intermediate (50°C): 0.06 kWh/m²/day
- Metal RISA: Low temp: 14,256 Btu/day, 368 Btu/ft²/day, ~4.2 kWh/m²/day; Intermediate: 2,067 Btu/day, 53.4 Btu/ft²/day, ~0.6 kWh/m²/day (low), ~1.2 kWh/m²/day (intermediate)
Comparison to conventional collectors (from FSEC sample ratings): glazed collectors in the 274–786 Btu/ft²/day range (intermediate temperature); RISA values are lower per unit area but achieve ~1 kWh/m²/day hot water energy in favorable conditions — similar order to PV solar conversion efficiency.
Water heating
- Glazed collectors are 10–15× more efficient (per unit area) than metal roof RISAs.
- Metal roof RISAs are ~10× more efficient than asphalt shingle RISAs.
Pool heating
- Unglazed collectors are ~2–3× more efficient (per unit area) than metal roof RISAs.
- Metal roof RISAs are ~2–3× more efficient than asphalt shingle RISAs.
- Roof-integrated solar absorbers can provide on the order of 1 kWh/m²/day of hot water energy (similar solar conversion efficiency to PV).
- For both hot water and pool heating, roof-integrated solar absorbers are an aesthetic alternative to roof-mounted solar collectors.
- They can also reduce solar heat gain in cooling-load-dominated climates.
Source: FSEC/NREL, U.S. DOE Building Technologies Program. Text from PDF.
PDF: 1999-01-01-roof-integrated-solar.pdf