Raising the Bar for Geothermal: How to Get Ultra-High Efficiency

Red Clover Commons in Brattleboro, VT. Courtesy photo.

Matt Desmarais and Jeff Harrison

Housing Vermont, a nonprofit development company, wanted to create fifty-five new, permanently affordable rental-housing units. To keep their costs permanently low, they worked with Blake Group to design and build a next-generation geothermal heating and cooling system for their construction project in Brattleboro Vermont. The primary energy-related goal of the project was to reduce the building consumption of propane and electricity to a point where usage per apartment was low enough that future hikes in energy prices would not have a significant impact on unit rental fees. Secondarily, for global sustainability, a low CO2 footprint was desired.

Over the past eight years, The Blake Group has been refining the design of what it takes to make a geothermal system operate with higher performance while also reducing installation costs, resulting in a number of outstanding projects. The Brattleboro project uses one seventy-ton, ultra-high efficiency magnetic bearing chiller providing both the heating hot water and cooling chilled water. The geothermal energy source or sink consists of twenty-four advanced geo-exchange closed loop bore holes, each 500 feet deep.

For ultra-high efficiency, geothermal systems, the Blake Group has developed a new set of design paradigms to produce some outstanding results. For the Brattleboro project the results are in: the system has performed with a seasonally varied coefficient of performance (COP) that fluctuates between 6.8 and 13, roughly two to four times better than the average residential geothermal installation.

While the Brattleboro apartment building has been measured to be an outstanding performer, there is also still room for improvement that may be obtained in more complete version of the Blake Group’s design. What is the secret to ultra-high efficiency? It’s not one big secret, but a lot of small points that add up to big savings. The keys are:

1. A near frictionless magnetic bearing “Thermal Care” chiller is used in place of a traditional scroll compressors, which operates at an average COP of about 7.2. This type of compressor uses about 50% less energy than scroll compressors, though are only practical for commercial, industrial or multi-family HVAC loads.
2. Low temperature heating water. Low temperature terminal heating devices were sized to use 95°F to 105°F hot water, and provides a significant portion, perhaps 20% of the energy savings.
3. Grundfos Magna3 ECM pumps move water through the facility with lower watts / GPM than the more commonly used variable speed drives with AC motors. Just this change in pump type saves about 8% of the annual KWh consumed by the pumps.
4. Energy recycling using waste heat from cooling, for heating. When the building requires BTUs for heat, it’s high efficiency chiller first uses the building chilled water air conditioning loop as an energy source, then blends in warmer geothermal source loop water into the air conditioning loop to prevent it from getting too cold. This method of operation generally occurs all the time the outside air temperature is either above 40⁰F outside or below 74⁰F, a very significant number of operating hours. The system recycles these BTUs, using the chilled water to cool down apartments which are south facing, while pumping those same BTUs to apartments that are calling for heat.
5. Pipes are sized to reduce the required pump horse power, just one pipe size larger than more traditional rules of thumb make a significant reduction in annual KWH
6. Thanks to the increased heat transfer of advanced borehole heat-exchanger piping, “Versaprofiles: GeoperformX”, material in the ground, the closed loop boreholes do not operate below 48⁰F, well above freezing, so losses normally associated with the use of propylene glycol are eliminated.
7. While not used in Brattleboro, a small portion of the building energy collected as low temperature thermal solar heat (55⁰F) can also significantly reduce the annual KWH used by a geo-exchange system.

The water room with the Thermal Care near frictionless compressor on the left and Grundfos Magna3 ECM pumps visible on the right. Image: Blake Group

The Brattleboro project is one of several ultra-high efficiency geothermal projects that have been driven by the Blake Group. Want to see first-hand how Blake’s expertise can work for your commercial building? On July 30th, Blake invites you to tour their certified Net-Zero Building in East Windsor, Connecticut. Please RSVP in advance.

Looking for geothermal for your commercial, industrial or multi-family building? Blake does informational presentations around the Northeast on “Near Net-Zero HVAC by Design” and “The Myths and Science of Geo-Exchange” which shares the industry’s best practices on geothermal design.

For more information or to RSVP to see Blake’s net-zero headquarters, please contact Matt Desmarais at matthew.desmarais@blakeequip.com.

Matt Desmarais is an industrial sales engineer with Blake Group. Jeff Harrison is a Professional Engineer for Blake Group who specializes in geothermal design.