Life cycle production and costs of a residential solar hot water and grid-connected photovoltaic system in humid subtropical Texas
Authors: Sweeney James F. , Pate Michael B. , Choi Wongyu 
Published: September 2016

Hot and humid climates with high solar radiation have the potential to offset residential building energy consumption with the application of solar hot water and photovoltaic electricity generation. However, costs, lack of incentives for the systems, and the need for proof-of-concepts continue to limit market penetration. The surplus of natural gas in certain areas of the United States, particularly Texas, continues to keep gas and electricity production economical compared to solar alternatives. However, trends that demand lower energy homes, a desire for local energy independence, and the lowering of carbon dioxide emissions continue to fuel solar energy systems penetration. To support solar use, this research was performed to evaluate and analyze the real-world life cycle energy and costs of a solar photovoltaic and solar hot water system (SHWS) installed on a high-end residential home in Houston, TX (IECC Zone 2). The house was a well-insulated, large urban home with two renewable energy systems installed: a 3.5 kW solar photovoltaic system (SPVS) and a 1.71 kW solar hot water system. Analyses were part of a larger study performed to investigate the contributions of the solar energy offsets on the operational energy of the building over a life cycle period of 30-years. Field measurements of energy production were compared to solar energy simulations based on the typical meteorological year and the National Solar Radiation Database (NSRDB) data. NSRDB provided the basis for a probabilistic interpretation of annual energy production in terms of probability measures, P50/P90. It was found that field estimates were within simulation uncertainties and P90 predictions were within 2.5% of TMY3 (typical meteorological year data 3) results for both the solar photovoltaic system (SPVS) and solar hot water system (SHWS). Additionally, optimizations in the system design and life cycle costs were investigated to determine the annual optimal performance for the solar energy systems. The SHWS was installed at a less than optimum azimuth of 270° instead of 180° (corresponding to a 16% reduction in annual output). The SPVS was installed at optimal design conditions of 180° azimuth and 42° tilt. Payback and levelized cost of energy (LCOE) could have been minimized with the addition of another solar hot water collector with a minimal impact to overall cost. Cost sensitivity analysis on the LCOE and net-present value (NPV) were also performed and over a 30-year life cycle period, TMY3 based simulations predicted a NPV of $796 (21.8-year non-discounted payback) and -$1246 (29.2-year non-discounted payback) for the SHWS and SPVS, respectively. The SHWS achieved a LCOE of 8.1 ¢/kWh, while the value for the SPVS was 12.29 ¢/kWh. For the SPVS, the photovoltaic module and collector costs were the largest life cycle costs, and of special note, a reduction in the module cost by 67% reduces the LCOE to the market average-high electricity price of 10 ¢/kWh. Finally, the combined renewable energy systems generated an estimated 30-year life-cycle energy production of 184 814 kWh, with auxiliary gas provided for supplemental hot-water heating. On average, a Texas residential home utilizes 420 000 kWh of electrical energy, 19% of which is domestic hot water demand.