No abstract

Executive SummaryThis paper examines the break-even cost for residential rooftop solar water heating (SWH) technology, defined as the point where the cost of the energy saved with a SWH system equals the cost of a conventional heating fuel purchased from the grid (either electricity or natural gas). We examine the break-even cost for the largest 1,000 electric and natural gas utilities serving residential customers in the United States as of 2008. Currently, the break-even cost of SWH in the United States varies by more than a factor of five for both electricity and natural gas (from less than $2,250/system to over $10,000/system for electric and from less than $1,000/system to approximately $5,000/system for natural gas, excluding Hawaii and Alaska), despite a much smaller variation in the amount of energy saved by the systems (a factor of approximately one and a half). The break-even price for natural gas is lower than that for electricity due to a lower fuel cost. It was found that for a $7,000 SWH system capital cost (electric auxiliary heater), break-even conditions currently exist in 73 electric utility service territories (serving 16% of all residential customers). To see similar economics for SWH systems with natural gas backup, the SWH system capital cost would have to drop to $2,500. We also consider the relationship between SWH price and solar fraction (percent of daily energy requirements supplied by the SWH system) and examine the key drivers behind break-even costs. Overall, the key drivers of the break-even cost of SWH are a combination of fuel price, local incentives, and technical factors including the solar resource location, system size, and hot water draw. v

A high-temperature, high-pressure solar receiver was designed as part of the advanced thermal energy storage project carried out in collaboration with Abengoa Solar NT at CSIRO Energy Centre in Newcastle, Australia, with support through the Australian Renewable Energy Agency (ARENA). The cavity-type receiver with tubular absorbers was successfully installed and commissioned, using concentrated solar energy to raise the temperature of CO2 gas to 750 °C at 700 kPa in a pressurized, closed loop system. Stand-alone solar receiver tests were carried out to investigate the thermal characteristics of the 250 kWt solar receiver. The on-sun full-load test successfully achieved an outlet gas temperature of 750 °C while operating below the maximum allowable tube temperature limit (1050 °C) and with a maximum pressure drop of 22 kPa. The corresponding estimated receiver thermal efficiency values at full flow rate were 75% estimated based on measured receiver temperatures and heat losses calculations for both single aim-point and multiple aim-point heliostat control strategies. The use of a quartz glass window affixed to the receiver cavity aperture was tried as a means for improving the receiver efficiency by reducing convective heat losses from the receiver aperture. However, while it did appear to significantly reduce convective losses, a more effective metal support frame design is necessary to avoid damage to the window caused by stresses introduced as a result of distortion of the supports due to heating by the spillage of rays from the heliostat field.