Solar Water Heating Report

This report fulfills the Part 1 of Task 2 in the Community-Scale Solar Water Heating Project scope of work. The Solar Hot Water Heating Report is the first deliverable of Task 2. The goals of this task are to review existing solar water heating technologies and systems, and establish the basic system design and construction requirements for community-scale solar water heating systems in Southern California. This report also assesses obstacles in applicable product, building, and land use codes for solar thermal systems. The report concludes with recommendations for appropriate component technologies and a basic system design for community-scale solar water heating systems in Los Angeles County. 1.1 Background Limited availability of natural gas and abundant sunshine made solar water heating (SWH) systems an attractive choice for consumers during the end of the 19th and early 20th centuries.1 Prior to the 1930s, a limited natural gas distribution network and high energy prices drove demand for domestic solar water heating systems. In 1897, one-third of homes in Pasadena had solar water heaters.2 In the next several decades thousands of additional units were installed in California.3,4 Consumers could heat water year-round without having to use a stove, saving fuel and keeping residences cooler during the summer months.5 During the 1930s, falling natural gas prices, urbanization, and incentives for consumers to switch to gas water heaters led to the displacement of solar thermal technology from the domestic market.6 Fluctuations in energy prices during the 1970s and 1980s had a modest positive impact on demand for solar thermal, but as of 2009 more than 90% of households in California have gas or electric water heaters.7 1.2 Natural Gas Consumption for Residential Water Heating California’s natural gas consumption patterns indicate that diminishing the amount of natural gas used for residential water heating is an effective strategy for reducing greenhouse gas emissions from the residential housing sector. Table 1 shows that 1 Florida Solar Energy Center. (2006). Solar Water and Pool Heating Manual. Solar Water and Pool Heating Manual, (January). Retrieved from 2 Islam, M. R., Sumathy, K., & Khan, S. U. (2012). Solar water heating systems and their market trends. 5 residential consumption represents approximately one-fifth of all natural gas deliveries statewide. Table 1: Natural Gas Deliveries by Consumption Category (MM ft3)8 Consumption Category Volume of Natural Gas Delivered (MMft3) Percentage of Total Gas Deliveries Residential 9,763,279 21.63% Commercial 4,915,750 10.89% Industrial 14,929,914 33.08% Vehicle Fuel 184,247 0.41% Electric Power 15,340,675 33.99% Total Deliveries 45,133,865 100.00% While water heating accounts for around 25% of total energy end use in residential buildings, water heating accounts for around 49% of residential natural gas consumption9,10. Additionally, 87% of residential buildings in California have gas water heaters, making residential water heating a major source of natural gas consumption.11 Figure 1 shows annual natural gas consumption in residential buildings for the three largest natural gas utility providers in California, broken down by water heating, space heating, and general base use.12 Figure 2 shows statewide residential natural gas consumption by end-use.13 The proportion of total residential consumption represented by water heating suggests that a transition to a renewable sources of heat can yield significant energy savings and reduce greenhouse gas emissions. 8 U.S. Energy Information Administration. (2017). California Natural Gas Consumption by End Use [Data set]. Retrieved from: 6 Figure 1: Residential natural gas consumption by for the three largest natural gas utilities in California.14 Figure 2: Residential natural gas consumption by end use15 14 Ibid. 15 Ibid. 7 1.3 Community Scale Approach to Solar Hot Water This report focuses on the energy savings and environmental benefits of community-scale solar water heating systems. In this context, “community-scale” describes both the size of the system and an adherence to a set of design principles. Community scale systems occupy an intermediate space between the domestic and utility scales. This report defines community scale systems as those able to meet the hot water demands of tens of residential buildings up to hundreds of residential units with greater that the minimum solar fraction required by law. Community scale energy systems are intended to make maximally efficient use of local resources where possible and create a range of options for residents to contribute to its operation. According to the CEC and National Renewable Energy Laboratory (NREL), community scale solar energy projects should include the following considerations.16, 17 • Primary Considerations o Make economically optimum use of local space and resources when and where possible. o Develop community scale energy infrastructure in a socioeconomically equitable manner. • Secondary Considerations o Improved economies of sale o Improved project siting o Exploration of new models for service delivery and project financing. A community scale approach to solar water heating in LA County is consonant with the considerations listed above. LA County has a mild, Mediterranean climate with abundant sunshine, but land use and development patterns range from densely populated urban areas to near-rural exurbs. In places where residents cannot afford to install separate domestic systems, or where space for system infrastructure is limited, a community scale approach offers opportunities for all participants to receive the benefits of solar water heating and support their system’s operation. Residents may contribute by allowing system infrastructure to be installed on their property, or by contributing financially if they do not own property on which collectors or tanks can be placed. Studies of solar district heating in Northern Europe suggest that there are positive returns to scale for solar water heating systems. Figure 3 shows that both the cost per unit heat delivered and system cost per collector diminish as the total collector area of a district solar heating plant increases. 16U.S. National Renewable Energy Lab. (2010). A Guide to Community Solar: Utility, Private, and Non-profit Project Development. Retrieved from: 8 Figure 3: Energy cost and system cost per collector vs. collector area for district solar heating systems.18 A community scale approach to solar water heating may be superior in terms of economic efficiency to the installation of many smaller domestic solar water heating systems. The proportion of the heat load supplied by solar energy, called the solar fraction of a system, depends on the amount of useful heat collected and the thermal losses from various system components.19 Larger systems require larger storage tanks, which store heat more efficiently than numerous smaller tanks, thus diminishing the cost per unit heat delivered.20 Furthermore, community scale systems distribute fixed costs among many users, allowing residents who do not have the financial resources to install their own solar water heating systems to enjoy low carbon hot water and reduce their consumption of natural gas.21 18 U.S. Army Corps of Engineers. (2011). Central Solar Water Heating Systems Design Guide. Retrieved from: 9 1.4 Benefits of Reduced Gas Consumption Reducing residential natural gas consumption will in turn reduce greenhouse gas emissions, diminish concentrations of local air pollutants (such as SO2 and NOx), and mitigate the likelihood of major fires and natural gas leaks. Given the share of natural gas deliveries consumed to heat water, the end use is an attractive target for reducing the greenhouse gas emissions from the residential housing sector. In 2016, California’s residential gas consumption for water heating totaled 201,795 million cubic feet, resulting in the emission of eleven million tons of CO2.22 This volume of carbon dioxide is equal to that emitted annually by an American city with a population ~700,000.23 Since water heating accounts for 25% of residential energy consumption, energy savings from an increased share of renewable heat are non-trivial. Reducing residential natural gas consumption will also reduce the probability and severity of a catastrophic failure of LA County’s gas delivery and storage infrastructure. Accidental releases of methane, both large and small, increase California’s emissions budget and make achieving its climate goals more difficult. The 2015 Aliso Canyon Gas Leak alone released 5 billion cubic feet of methane in 112 days, equivalent to the annual emissions of ~600,000 cars.24 The storage and transmission of methane leads inevitably to releases that increase California’s emissions footprint. In addition to the environmental costs of leakages, a large earthquake in Los Angeles County could ignite numerous gas-fueled fires. A study by the California Seismic Safety Commission estimated that 20-50% of fires resulting from a major earthquake (M > 6.0) will be caused by the ignition of natural gas leaks.25 Reducing residential demand for natural gas will reduce the volume of gas that must be stored and delivered, mitigating the risk of leakages and fires.