Renewable Hot Water Systems: Solar Collectors and Heat Pumps

Topics: Air Source Heat Pump, heat pump, renewable heating, solar hot water, solar collector

Chelsey Bipat
Author : Chelsey Bipat on March 12, 2020

Water heating represents a significant energy expense in buildings, especially in the residential sector. Many commercial buildings such as restaurants, hotels and healthcare facilities also use plenty of hot water. The two traditional methods to heat water have been combustion and electric resistance, but solar collectors and heat pumps provide a greener alternative.

Combustion heating has a low operating cost, but fossil fuels are burned at the point of use. In addition to having a negative environmental impact, combustion heating reduces air quality in urban settings. On the other hand, electric resistance heaters produce no direct emissions, but their operating costs are very high. In addition, if the local grid uses fossil fuels as the main power source, resistance heating simply moves emissions from buildings to power plants.


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Solar collectors use a free resource that reaches the point of use by itself - sunlight. There is only a small pumping cost when solar collectors are installed on rooftops or other high places. Heat pumps use solar energy indirectly, since they heat water by gathering thermal energy from the outdoor air. Heat pumps run with electricity like resistance heaters, but energy consumption is reduced by 50% or more.

According to the NYC Urban Green Council, hot water accounts for 10% of overall energy consumption in buildings. For multifamily buildings in particular, hot water represents 19% of energy consumption. Renewable heating methods can lower the environmental footprint of these buildings, while lowering their energy bills.

Solar Collectors and Heat Pumps: Comparing Their Savings

Solar collectors and heat pumps both achieve energy savings, but they differ in how these savings are achieved.

  • Solar collectors are exposed directly to sunshine. They use antifreeze solution or another heat transfer fluid to gather thermal energy, and then a heat exchanger is used to heat water without mixing. In tropical locations with hot climate, solar collectors can be designed to heat water directly without an intermediate fluid.
  • Air-source heat pumps gather thermal energy from the outdoor air, which means they can run at night and they don’t need direct sunlight. Actually, heat pumps can gather energy from outdoor air even during the winter. However, they become less efficient as the air temperature drops, and they must use a defrost cycle to remove ice from outdoor units.

Solar collectors cannot produce hot water 24/7, since they depend on sunlight just like solar panels. On the other hand, a heat pump can use thermal energy from outdoor air at any time. The two technologies are not mutually exclusive, and they can be deployed together to achieve greater savings. The solar collector maximizes free water heating with sunlight, while the heat pump meets the hot water demand that can’t be covered by the solar collector.

airsourceheatpump

Heat pump water heaters can achieve synergy with onsite renewable generation systems. Depending on its type and efficiency, a heat pump produces between 2 and 6 kilowatt-hours of heat for every kWh of electricity consumed. This means an energy output of 100 kWh from solar panels or wind turbines can be converted into 200-600 kWh of water heating.

Heat pumps can also be used as energy storage systems when there is surplus production from renewable sources. They can convert surplus electricity into thermal energy stored in water, and an insulated tank accumulates hot water for later  use.

Using Solar Collectors and Heat Pumps in New York City

In New York City, Local Laws 92 & 94 require sustainable roofing systems in all new roofs and existing roof extensions with at least 200 square feet. Only solar panels and green roofs count as “sustainable roofing systems” under the laws, but areas covered by solar collectors are exempt from the requirement. In other words, solar collectors can be used to reduce the roof area covered by LL92 & LL94. When deciding between solar collectors or photovoltaic panels, the best recommendation is contacting an energy consulting firm, to analyze the costs and savings of each option.

solarcollector

Roof areas used for mechanical equipment are also exempt from LL92 & 94, and this includes the outdoor units of heat pumps. A building can combine solar panels, solar collectors and heat pumps to save energy, and this does not conflict with the requirements in LL92 & 94.

Air-source heat pumps are a great option for electricity consumers with no roof space for solar panels or solar collectors. Their outdoor units can be mounted on walls, just like the condensers of mini-split air conditioners. Heat pumps are also a great option when the available space is covered with shadows, since they don’t need direct sunlight. On the other hand, solar panels and solar collectors become improductive when covered by shadows.

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