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| Fig. 1: Solar Water Heating in Weihai, China (Source: Wikimedia Commons) |
Solar thermal energy is a form of energy and a technology for harnessing solar energy to generate thermal energy. A widely known and increasingly commercial solar thermal application is solar water heating (SWH). [1] SWH applications use dedicated solar collections to draw power from the sun, which is then used to heat water. In terms of installation and energy costs over the lifespan of solar water heating systems, SWH technology has proven to be cost efficient across many different applications and settings. [1] SWH technology has the potential to reduce household water heating electricity consumption and also provides external benefits of reduced emissions of CO2 and other pollutants. [2]
Solar water heating (SWH) system usage has grown to a global capacity of 185 GW in 2010. [1] Though China leads in the SWH market (see Fig. 1), significant increases in adoption rates has also occurred in the European Union, Japan, India, and Brazil. [3] As of 2010, China, Germany, Turkey, Brazil, and India lead the solar hot water market. [1] On the other hand, the US domestic market capacity is still relatively small compared to the rest of the world. The SWH market is also expanding in Ethiopia, Kenya, South Africa, Tunisia, and Zimbabwe. [4] The rollout and adoption of SWH has evidently had varied success across the world.
Of the 110 million households in the United States that use fuel for water heating, 39% use electricity and 54% use natural gas. [2] Additionally, the national average water heating demand is 2814 kWh per house per year, of which 39% is electricity usage. [2] Thus, the national average water heating demand in the form of electricity is approximately 1,097 kWh per house per year. We can apply this number to the 110 million households to find that the electricity usage for water heating is approximately 121 billion kWh in the United States per year. Assuming the electricity is provided by natural gas, the conversion factor is:
| 3.6 × 106 J kWh-1 5.5 × 107 J kg-1 × 0.6 |
× ( | 44 16 |
) | = | 0.30 kg kWh-1 |
| 1.21 × 1011 kWh × 0.30 kg kWh-1 | = | 3.63 × 1010 kg of CO2 | |||
Similarly, the share of the aggregate residential energy utilization for other countries is about 25% in Australia, 22% in Canada, 14% in Europe, 37% in South Africa, 27% in China, and 29% in Mexico. [4] Heating comprises the largest share of energy supply, accounting for approximately 50% of world energy demand. [5] This energy supply is mainly generated from fossil fuels, which emit a considerable amount of carbon dioxide into the atmosphere, as seen in the above calculation. This high universal demand for water heating presents a unique opportunity to apply solar-based water heating solutions. Given how many metric tons of CO2 emissions traditional water heating appliances generate, there exists an environmental incentive for the adoption of efficient solar-based water heating appliances globally.
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| Fig. 2: United States Monthly Electricity Prices by Utility Service Territory, 2013 [7] (Source: Wikimedia Commons. Courtesy of the DOE) |
Studies suggest that current solar water heating technologies in the U.S. have the potential to save up to 85% of the energy used in conventional watering heating systems. [2] This corresponds to an annual savings for systems currently using electricity in the range of under 1600 kWh to over 2600 kWh for a typical household. [2] In the United States specifically, the limited success of solar water heating solutions includes limited retailer presence and perceptions regarding aesthetics and reliability. [2] Research further shows that a primary driver behind the low adoption rates of solar water heating remains the high upfront capital cost. [2] Moreover, benefits such as reduced use of fossil fuels and carbon dioxide emissions are an external benefit to the customer and prove difficult to precisely quantify. [2]
The quantification issue remains a constraining factor in the case of widespread adoption of solar water heating solutions. In the United States, research finds that a substantial increase in natural gas prices would need to occur in most of the United States to achieve break-even conditions on a $7,000 solar water heating system. [6] If natural gas prices doubled (from the 2008 baseline), only 25% of the residential energy demand would be in utilities where break-even conditions exist for a $7,000 system. [6] The only areas where a SWH system is at break-even or better is where there is a combination of high electricity prices, good solar resource, and local incentives. [6]
Since estimates of future electricity prices are uncertain and vary geographically (see Fig. 2), break- even analyses fluctuate over time and cast uncertainty onto the conditions under which solar water heating systems would realize savings for consumers. Given that break-even cost is a function of many variables such as solar resource, local electricity rates, hot water usage, and available incentives, there can be considerable variation in break-even cost across the United States. [2] SWH systems that replace conventional electric systems are more likely to break even in areas with either higher electric prices (e.g., in the Northeast) or high solar resources (in the Southwest). [2] Very low electricity prices and moderate system performance will obstruct break-even conditions, since the economic case for solar water heating adoption becomes unclear when certain weather or market conditions are not met. [2] Under certain conditions, solar water heating can certainly outperform fossil fuel-powered heating solutions. However, cost-benefit analyses are best conducted on a case-by-case basis since solar water heating costs vary considerably with weather conditions, the complexity of solar thermal installation, and costs of labor and materials. [5]
While solar water heating continues to generally present cost-effectiveness and has myriad successes across the Middle East, North Africa, China, and Europe, its case for adoption boils down to the alignment of incentives on the part of consumers. Financial incentives can be deployed as a tool to encourage retail consumers to utilize renewable energy sources to meet heat demands through the form of grants, operating grants, and soft loans. [5] Many countries presently provide capital grants or tax credits for solar water heating investments, which typically cover 20-40% of a systems costs. [5]
The patchwork adoption of solar water heating globally is more broadly representative of the intricate break-even analyses that underpin successful and widespread adoption of renewable energy-powered appliances across residential, commercial, and industrial sectors. Solar water heating demonstrably heats water as effectively as natural gas-fueled systems; however, due to the wide variability in its performance geographically depending on electricity prices and weather conditions, it is not a one- size-fits-all solution. In order to further reduce the friction associated with consumer adoption, many technical barriers behind solar water heating are being studied, such as the nonexistence of a universal certification approach to specify the standards for production and utilization, high heat losses at night, protection of solar collectors from freezing in cold temperatures, and lack of trained and competent installers. [5]
© Danny Valdez. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. R. Islam, K. Sumathy, and S. U. Khan, "Solar Water Heating Systems and Their Market Trends", Renew. Sust. Energy Rev. 17, 1 (2013).
[2] H. Cassard, P. Denholm, and S. Ong, "Technical and Economic Performance of Residential Solar Water Heating in the United States," Renew. Sust. Energy Rev. 15, 3789 (2011).
[3] R. Shukla et al., "Recent Advances in the Solar Water Heating Systems: A Review," Renew. Sust. Energy Rev. 19, 173 (2013).
[4] A. Gautam et al., "A Review on Technical Improvements, Economic Feasibility and World Scenario of Solar Water Heating System," Renew. Sust. Energy Rev. 68, 541 (2017).
[5] Z. Wang et al., "Solar Water Heating: From Theory, Application, Marketing and Research," Renew. Sust. Energy Rev. 41, 58 (2015).
[6] H. Cassard, P. Denholm, and S. Ong, "Break-even Cost for Residential Solar Water Heating in the United States: Key Drivers and Sensitivities," U.S. National Renewable Energy Laboratory, NREL/TP-6A20-48986, February 2011.
[7] R. Cruickshank et al., "Characterizing Electric Grid System Benefits of MPC-Based Residential Load Shaping," National Renewable Energy Laboratory, NREL/CP-5D00-71539, September 2018.
