Tuesday, December 25, 2007

Store Thermal Energy for Maximum Efficiency whether on the grid or off

I've been asked to put forward an energy saving concept for a small community in the coastal region of North Carolina. The idea is to design a system which would prove economical when using conventional energy, and to make it adaptable to renewable energy in the future. Natural gas is not available in the area, so the energy source defaults to electricity. In the past, this might have led to high utility costs. But with current electricity prices, driven by efficient power plants and low cost energy sources such as natural gas, coal, nuclear or hydroelectric sources, along with technology and a thoughtful design, it is possible to meet all the objectives.



The keys to success are heat pumps and energy storage. With today's highly efficient lighting and appliances, the large majority of our home energy use is for heat and cooling, including HVAC, hot water and refrigeration. And, while energy storage in batteries is quite expensive, energy storage in the form of heat is relatively cheap and easy.



By using a typical heat pump, we can generate relatively efficient heating and cooling. By using the more moderate temperatures available from the earth or a pond, we can improve the efficiency considerably. But, we would improve the efficiency even more if we could use both the hot and cold side of the heat pump. Unfortunately, heat load and cooling load rarely match.



That is where energy storage comes in. By capturing both the heat and cooling in water storage tanks, we can more easily match heat and cooling load, and utilize the ground source only for the longer term load differential. The energy storage not only allows us to use both the hot and cold side of the heat pump, but it allows us to take advantage of off peak rates and to better utilize renewables, since energy storage is one of the most difficult issues in each case.



Fortunately, it is relatively simple to implement a system with all these advantages. In concept it would look something like the below.
Start with a compressor, perhaps similar to the one on your car air conditioning system. The compressed refrigerant goes to the darker red coil, where water circulates the heat generated to the hot storage tank. Then the refrigerant flows to the lighter red coil, where it can be further cooled by circulating water from the ground source to maximize cooling. The refrigerant goes through an expansion valve to reduce pressure and generate cooling, then flows to the darker blue coil. Water from here is circulated to the Cold Storage. Then to the lighter blue coil, where ground source water is circulated to maximize heating. From there, it is back to the inlet of the compressor, where the process repeats itself.

As a result, you have an extremely efficient system for generating the heat and cooling required by your house. The storage allows matching of your cooling and heating load over time, and also allows greater utilization of off peak power. Best of all, it can even out the swings associated with wind and solar power. The compressor can be turned either by electricity, or directly by the wind turbine. And the same storage allows you to utilize solar heat collected during the day, and cooling captured in cold evenings with the solar collector. Then just circulate the water to sypply heat or cooling needs in the house.

You'll still need electricity for lighting, appliances and electronics, but the combination of the above system and energy efficient equipment available today can dramatically decrease the amount required. And, if the desire is to live off grid, the electricity storage requirements are minimized to the point where they can be met at a reasonable cost by batteries. And, the above system could be electricity free if the equipment is arranged to allow thermosiphoning, rather than pumping of the water.

I'm open for comments.

6 comments:

ASG said...

Energy Guru - what's the approximate up front cost of installing said heating/cooling conservation system on top of the solar/wind sources for power generation? And then what's your estimated payback period?

I've considered a more conventional ground source heat pump system for installation in my future dream/green/retirement home, and would be interested in knowing more about the $$ impact for this type of system.

Thx.

ASG
www.hopsandbarleyblog.com

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