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Ground-Coupled Heat Pump and Energy Storage

©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 4, April 2013.

By Ed Lohrenz, Member ASHRAE; and Sergio Almeida, P.Eng., Member ASHRAE

About the Authors
Ed Lohrenz and Sergio Almeida, P.Eng., are partners in Geo-Xergy Systems Inc., in Winnipeg, MB, Canada.

Ground-coupled heat pump (GCHP) systems consume less purchased energy than an HVAC system using fossil fuel and electricity directly for heating and cooling. However, the cost of building the ground heat exchanger (GHX) often prevents acceptance of GCHP systems.

The incremental cost of a GCHP is driven primarily by the cost of building the GHX. The size of the GHX needed for a project is determined by four factors: peak heating and cooling loads, annual heating and cooling energy loads, geology, and configuration of the GHX.

A designer has no control over the geology or size and configuration of the site. The GHX must be designed for the project. A designer can have some control over the building and systems. The purpose of this article is to illustrate how integration of thermal energy storage (TES) with a GCHP system can reduce the cost of a GHX and reduce energy cost. Hypothetical examples based on actual projects illustrate the integration of a GCHP system with TES to produce domestic hot water or provide cooling more cost effectively and efficiently in certain types of GCHP projects.


Energy Storage

You very likely took advantage of TES during your morning shower. A hot water tank with 10,000 Btu/h (3 kW) heating capacity, heats 11.5 gallons (43.5 L) of water for your five-minute shower, using 5,750 Btu (1.65 kWh) of energy. An on-demand water heater would require a 69,000 Btu/h (20.2 kW) element to heat 50°F (10°C) water to 110°F (43°C) to supply 2.3 gpm (0.14 L/s).

If your teenager wants a shower at the same time, you will need a second 69,000 Btu/h (20.2 kW) on-demand water heater. But if you have a storage tank full of hot water, the same 10,000 Btu/h (3 kW) heater would work for just over an hour to provide hot water for both showers before you head out.

If no one else uses hot water for a few hours, a 30 gallon (113 L) tank easily meets your hot water needs using a 10,000 Btu/h (3 kW) heater instead of a 69,000 Btu/h (20.2 kW) heater.

Energy storage also has implications on the design of the energy supply to your home. Wires connecting your home to the grid, electrical panel, circuit breakers and size of the wires to your water heater are all affected. It may have implications on the size of your electricity bill if your home has a smart meter measuring peak electrical demand as well as consumption.

TES has implications for your electric utility. The size of generating stations, distribution grid and transformers needed to get energy to your home, and with some utilities even the type of fuel used to generate electricity, are all impacted by your decisions.


Domestic Hot Water Storage Tanks in a GCHP Application

Many facilities have predictable and intermittent domestic hot water (DHW) requirements. Occupants in apartment buildings tend to use hot water heavily for a few hours in the morning before heading to work and a few hours in the evening preparing and cleaning up after dinner.

When designing a DHW system using a gas boiler or gas water heaters, there is little impact on the capital cost on the system. It may cost less to increase the capacity of the boiler and decrease the storage tank size when space in the mechanical room is limited. The following example illustrates the impact of TES on a GCHP system.

A typical apartment building was selected to illustrate the impact of DHW production using a large heat pump/small storage tank compared to a small heat pump/large storage capacity combination. The following assumptions were used to calculate heat pump and storage tank capacity (Table 1).


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