Since Roman times, humans have used wheels to harvest energy. Water wheels, like those associated with historic sawmills, harnessed kinetic energy. Mills strategically dammed flowing waterways to take advantage of the natural tendency for water to flow downhill. Then, water was forced through a narrow “race” that directed the flow of liquid over the wheel. The spinning waterwheel transferred the kinetic energy of the moving water into a functional process, powering machinery inside the mill.
Heat, another form of energy, can also be harvested using large wheels. Alternatively called enthalpy or heat recovery wheels, these devices harken imagery of water wheels to the novice eye because of their similarity in size and rotational speed. But, unlike the water wheels of yesteryear, these wheels are far more likely to be found deep in the mechanical control space of a building than on a scenic waterway.
Buildings consume nearly half of the energy used in the US each year. A significant portion of this energy is used for indoor climate control. Enthalpy wheels have the potential to dramatically reduce the energy consumption associated with heating and cooling because they are designed to “recycle” the temperature qualities of exhaust air, as shown in the diagram above. These wheels have a significant upfront cost (in the hundreds of thousands). But, the payback on investment for commercial spaces can come within years or even months. Cost savings come both upfront, because smaller-scale heating and cooling units can be purchased, and downstream through reduced utility bills.
The most efficient enthalpy wheels are typically constructed of aluminum. Both exhaust and intake air are directed into the wheel. The interior of the wheels have a honeycomb structure, a structural design that dramatically increases the surface area over which air can exchange. This structure is typically coated in silica, which has moisture transfer properties that help to maintain comfortable levels of humidity for indoor air.
For homes, and other small spaces without dedicated maintenance staff, a fixed-plate heat exchanger can provide similar energy saving benefits. These small lightweight boxes (costing a few hundred dollars) are commonly installed as part of the ventilation system of modern green homes in order to overcome the air quality issues associated with building air-tight structures.
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