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Renewable Heating And Cooling

Renewable Hot Water Heating

About Hot Water Heating

Hot water is an essential part of everyday life across the United States. In homes, domestic hot water is used for showering, hand-washing, laundry, dish-washing, and other functions. In 2009, delivered energy for residential water heating accounted for nearly 2 quadrillion Btu, or roughly 18 percent of total energy use in U.S. residences.1 In 2003, commercial settings such as car washes, laundromats, and commercial kitchens used more than 500 trillion Btu for water heating, which was nearly 8 percent of their total fuel consumption.2

Related applications described separately include pool heating and hot water for industrial applications such as chemical production.

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How Renewable Hot Water Heating Works

Water heating in most residential and commercial buildings simply requires a source of heat and a network of pipes to deliver hot water to the point of use. Water heating systems fall into two main categories:

  • Batch heating. Batch systems apply heat to water stored in a holding or storage tank.
  • On-demand heating. On-demand “tankless” heaters run cold water directly through a heat source. The water is quickly heated en route and delivered hot. These systems tend to be more energy-efficient than batch heating, but they also have a larger up-front cost.

Renewable sources can support both of these types of water heating systems. The renewable heat source serves to preheat the water, with a conventional unit kicking in to make up the difference when the renewable heat source cannot accommodate the entire heating load—for example, a cloudy day when a solar system’s output is reduced, or during a time of increased demand.

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Compatible Renewable Technologies

Among solar technologies, flat plate and evacuated tube solar collectors are most commonly used for hot water heating, particularly in residential settings. These technologies are scalable, so large loads can be supported if enough collectors are installed. The major constraints on effective solar water heating are the upper temperature limits (see diagram below) and the amount and quality of sunlight available. For example, at high latitudes in the winter, evacuated tubes will tend to capture low-angle sunlight more efficiently than flat-plate collectors. A detailed analysis of your building site will inform the system developer how many solar hot water collectors and how much hot water storage capacity will be needed to meet your hot water needs. Because solar water heating systems can meet on average 65 percent of your hot water use, you will still rely on your conventional heating system as a backup.

Geothermal direct use and deep geothermal steam can also support very large water heating loads. These are relatively less common options given the geographic availability limitations. Ground source heat pumps, however, are available anywhere in the United States and are highly effective as a preheat for domestic hot water when designed as part of a larger space heating and cooling system.

Woody biomass can be burned in place of fossil fuels to heat water, or sometimes as part of a combined heat and power system for an industrial or institutional facility.

The interactive diagram below shows which renewable technologies can serve water heating applications. You can click any of the technologies to go to a new page with more detailed information.

Renewable Hot Water Heating Technologies and Applications

Understanding the Diagram

The diagram above shows technologies and water heating needs in terms of the approximate “working temperature” range, which is the required temperature of the heat transfer fluid within the renewable heating system. The working temperature is not necessarily the same as the final temperature of the end product (in this case, the final temperature of the water that comes out of the hot water heater). For example, some water heating systems require a working temperature of more than 150°F, even if the system is only heating water to around 120°F.

The diagram above shows approximate working temperature ranges. The exact working temperature requirements for a particular system will depend on factors such as system type, size, and location. The working temperature that a particular renewable technology can supply will also depend on site-specific factors. For example, the amount of heat that a solar collector system can supply will depend on how much sunlight it receives, and at what angle.

U.S. Energy Information Administration. 2012. 2009 Residential Energy Consumption Survey. Table CE3.1 Household Site End-Use Consumption in the U.S., Totals and Averages, 2009.
U.S. Energy Information Administration. 2008. 2003 Commercial Buildings Energy Consumption Survey. Table E1A. Major Fuel Consumption (Btu) by End Use for All Buildings.

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