A year in green tech: Energy recovery ventilation

I live in a one-hundred-plus-year-old house.  Even with some attention to sealing up gaps and replacing windows, I still have drafty spots where cold air leaks in during the winter and warm air leaks in during the summer. 

And I am ok with that.

Truth is, we need fresh air in buildings just as much – if not more – than we need thermal comfort.  We have mechanisms in our body and choices we can make to cope with feeling warm or chilly, but if we have poor air quality indoors, then we have no manner to cope with that individually.

That fresh air comes at a cost, however.  Heating and cooling fresh air can cost as much as fifteen percent of a building’s energy budget.  Because of this, in the early 1980s, the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE), the organization that establishes standards for building system performance, reduced the acceptable level of outside air required to ventilate buildings.  The thinking held that because we needed to improve energy performance, if we lowered the amount of outside air, we would lower the cost.  It worked, but immediately, we saw a rise in complaints about sick-building syndrome.  The lower ventilation levels could not keep the indoor air fresh enough to maintain occupant health, and within a decade, the levels returned to their previous level.

Flash forward to today where we have made great strides at reducing heating costs through insulation and electricity costs through better equipment and lighting, and ventilation air consumes an even larger chunk of our energy.  We have strategies such as demand control ventilation to mitigate the costs; in demand control ventilation, we monitor the occupants in the space, and if the number drops below the expected threshold, we reduce the amount of air we deliver.  This only works when you have the system to monitor, and when the building exhausts an amount of air significantly less than the amount needed for ventilation.

Popular Mechanics

Fortunately, we have a technology to reduce energy while keeping ventilation high that over the past years has dramatically dropped in price and improved in performance:  energy recovery ventilation.  The process works  on a simple principle that requires some complex application.  In the winter, when outdoor air sits at 0F to 20F, our buildings generally maintain indoor temperatures at 68F to 73F.  Adding to this the heat gained from lights and equipment, and the air that we exhaust from our buildings could have a temperature of 75F to 78F.  Without the concern for energy, we would just throw that air out of the building.  However, that air contains valuable heat energy that will benefit us if we can transfer it to the incoming air.  Similarly in the summer, we have building air at 75F, and more importantly with a relative humidity near 50%, and outdoor air as warm and moist as 95F with as much as 80% relative humidity.  In this case, the exhaust air could absorb the heat and moisture from the incoming air.

This process requires complex planning and materials development.  We need to determine the type of energy we want to transfer: do we want to change just the temperature or do we want to move moisture as well?  If we only want to transfer heat to change temperature, we can get away with a setup that passes both air streams through a highly conductive metal enclosure that isolates the air molecules from each other, but allows the heat to move.  If we want to transfer the moisture as well, we need a wheel with specially-designed coating material that absorbs water from one air stream and passes it into another.  Also, we need to plan the path for our air flows so that the exhaust air passes near our incoming air.

Dessicant and enthalpy (or total energy) wheels perhaps have the most interesting characteristics.   The surface material of the wheel has microscopic “holes” in the surface specifically designed to pull water vapor from one air stream.  When exposed to the less-humid stream, they then release that water.  Such a highly specialized surface requires a precise manufacturing process, and originally the cost of this exceeded the value delivered.  With recent developments in technology, however, the costs have declined to the point where energy codes now require these types of wheels in certain applications.

Not all exhaust streams work with total energy or dessicant wheels.  Toilet exhaust, for example, has contaminants that we do not want transferred to the incoming air.  In those cases, we may use the sensible heat transfer wheel, or a simple air-to-air heat exchanger.   In that way, we can at least recover the heat while avoiding the contamination that can come from taking the moisture from one air stream and transferring it to the other.

Depending on the balance between exhaust air volumes and the needed ventilation air, energy recovery can significantly reduce the cost of treating outside air.  If the exhaust and outside air rates nearly match, we can lower costs by as much as 50%.  In some applications, we can reduce the cost even further.

Not every form of green technology comes from the sun or some newly developed electronic gadget.  Sometimes, green tech simply looks for untapped energy sources right under our noses, then efficiently transfers that energy on a novel way.  ERV is just such a technology: not fancy or exciting, but effective.