Energy storage has promising applications in buildings and clean energy generation. Although the media has recently focused on lithium-ion batteries, there are other viable forms of energy storage, and one example is heat storage. Given that space heating and hot water represent a large portion of energy consumption in U.S. buildings, opportunities for utilizing heat storage are abundant.
A heat storage system can be as simple as an effectively insulated water tank, as water can retain large amounts of heat in a compact volume. When a cubic meter of water reduces its temperature by 10°C, it releases 41,870 kilojoules of heat – equivalent to 39,685 BTU. Hot water can be used directly or can provide space heating by circulating through air handlers or radiant heat pipes.
Design an ideal heating system for your construction project.
Hot water storage applications
When heating systems use natural gas or fuel oil, their operating cost is not affected by time of use – heating 100 gallons of water costs the same at any time. However, combustion heating produces local emissions, and clean energy electric heating is among the alternatives proposed to reduce pollution from buildings.
Electric heating is sensitive to time of use, unlike combustion heating, and its use can become very expensive when not managed properly:
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Energy consumers with time-of-use rates pay a very high price per kilowatt-hour during peak demand times.
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Large consumers with demand charging are charged for their largest consumption peaks, in addition to the normal charge for the amount of energy used.
Electric resistance heating has been around for decades, but its operating cost is high because each kilowatt of heating production requires a kilowatt of electricity. On the other hand, electric heat pumps provide between 2 and 6 kilowatts of heat for every kilowatt of electricity. The coefficient of performance (COP) depends on the specific type of heat pump, its design and its operating conditions:
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Ground source heat pumps are more energy efficient than air source heat pumps.
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Heat pump efficiency is gradually reduced as the outdoor temperature drops.
Space heating and domestic hot water have a minimal environmental impact when using heat pumps powered by low-cost electricity from wind turbines and solar farms. The main disadvantage of this configuration is the variable output of solar and wind energy systems, but hot water storage can balance energy supply and heating needs.
As electricity is much easier to transport than water, the ideal location for storing hot water is close to the point of use. When a hot water tank is far from the point of use, pumping can cause a significant increase in operating costs. Also consider that longer pipes lead to greater heat losses.
Utility-Scale Heat Storage
Distributed heat storage is a viable enhancement for space heating and domestic hot water systems. However, heat storage has also been tested at utility scale alongside concentrated solar power (CSP).
Unlike a photovoltaic array, which uses solar modules, a CSP installation concentrates sunlight with mirrors. The intense localized heating is enough to drive a steam turbine and generator:
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The concentrated heat melts a special salt, usually sodium nitrate or potassium nitrate.
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The molten salt travels to a heat exchanger, where it boils water into steam to drive turbines.
With this configuration, molten salt is stored in refractory containers, where it retains heat for long periods. This way, the system can continue producing steam for the turbines even when there is no sunlight. Although PV arrays offer a lower electricity cost than molten salt CSP systems, they cannot provide electrical power on demand.
Unlike photovoltaic arrays, CSP stations require a large scale to be economically viable. For a building owner looking to maximize the use of renewable energy, a solar photovoltaic system with energy storage offers the flexibility to scale down. CSP stations for self-generation are possible for users reaching the megawatt scale – large industrial parks would be viable candidates.
This concept is still relatively new and has only been implemented in a few power plants around the world. The first commercial application of CSP with heat storage in the US was the 110 MW Crescent Dunes plant in Nevada.
Conclusion
Heat storage enables greater use of clean energy in space heating and domestic hot water applications, which traditionally rely on combustion. Hot water storage can achieve synergy with heat pumps, which provide between 2 and 6 kWh of heat for every kWh of electricity consumed. Although heat storage can also be used with conventional resistance heaters, the system becomes inefficient and expensive to operate.
Molten salt has been successfully used as a heat storage medium in large-scale generation, as it can produce steam even when a heat source is not available. Molten salt has been used to provide nighttime generation in concentrated solar power plants and can also capture heat from compressed air energy storage – another emerging technology.
