You May Be Standing Atop a Valuable Renewable Resource

Oct. 15, 2021
Geothermal heat pumps address six microgrid imperatives.

Depending on your point of view, distributed energy resources (DER) will evolve in a number of directions. At a large energy conference in 2017, one particularly animated speaker touted the imminent launch of solar paneled space kites. While that seemed a stretch, just down the hall a petroleum executive described a future with natural gas turbines in every neighborhood. It’s probably fair to say neither of those scenarios is realistic. The real future of DER likely will be found somewhere in between the two extremes.

One thing most experts agree on, however, is that microgrids will play a role in the future of energy distribution. How many meters will be on a microgrid in 2030 is hard to predict. Nevertheless, it’s a good bet that there will be quite a few more than today. With that in mind, here are six microgrid paradigms with corresponding design imperatives. Simply put, microgrid life will change priorities, and new priorities need different solutions.

1. Power is Precious. The act of wasting power from the macrogrid goes unnoticed in most homes; we shrug and maybe remind the kids to turn off the lights when they leave the room. “Hey, do you think we’re made of money?” In reality, however, if homeowners need clean dishes, the dishwasher will run at 5 p.m. Even if the kWh rate is 10 or 20 cents higher.

Life in a microgrid, however, is more likely attuned to the financial and opportunity costs of power. If turning off an unused light fixture means the storage system will have enough power to run the freezer for the rest of the night, that’s a very different decision than spending the 20 cents to leave a light on.

So, if microgrid power is a more valuable commodity, then efficient use of power is the natural imperative. “Waste not, want not,” as Granddad used to say.

Cutting straight to the heart of the matter, half of household energy budgets typically go toward heating, cooling, and hot water. Put another way, half of every home’s energy is not electrical; it's thermal. And this is where a geothermal heat pump makes sense, using small amounts of electrical power to deliver four times as much heating, cooling, and hot water. This is taking roughly half of a building’s energy consumption and reducing it by half or more.

Crazy you say? Not possible? Well, DoE ratings for geothermal heat pumps (GHPs) certify that they deliver at least twice—and up to five times—as much energy as they consume. Even more engaging is that GHPs are unaffected by the weather. While air source heat pumps lose over 40% of capacity and/or efficiency at 0°F or 100°F, GHPs remain tied to the constant temperatures below the surface of the earth.

2. Onsite Sourcing is Best, and what could be more “onsite” than the site itself? To reduce pipelines, wires, and fuel trucks tied to a microgrid, then onsite energy is the best option. When the weather is right, wind turbines are good and necessary. The same is true for solar energy systems. However, half of residential energy demand is thermal and can be pulled from the earth 24/7/365. GHPs harvest renewable thermal energy day or night, rain or shine, and on days when the wind doesn’t blow. Geothermal heat pumps deliver renewable energy for hot water, heating, or cooling…without interruption for decade after decade.

3. Electricity is the Highest Form of Energy; therefore, only use electricity when necessary. Electricity is necessary to charge phones and light homes for the foreseeable future, but half of total household energy is still thermal energy. Let’s put this in perspective: A family of four taking hot showers would typically use 16 kWh through a 100% efficient (COP 1.0) electric water heater. That same hot water would be produced by a GHP using only 4 kWh. When one considers the incremental cost of solar or wind power required for one solution or the other, the choice of 400% efficiency versus 100% is fairly straightforward. So, use electricity when necessary and use it sparingly.

4. Disruptions are Your Problem; therefore, it’s always best in a microgrid to maximize reliability. Life on the macrogrid includes the luxury of near instant response to outages by linemen and bucket trucks. Unless a microgrid is fully resourced with 24/7 staffing and repair infrastructure, equipment failures are a more personal—and potentially more challenging—event.

Building a microgrid for maximum reliability with GHPs just makes sense. Underground heat exchangers connected to a GHP typically should last over 100 years without material maintenance. Similarly, GHP aboveground equipment lasts 24–30 years with basic annual maintenance. That’s more than twice as long as the DoE expects air source heat pumps to last.

5. Storage is Good seems like an obvious paradigm for life in a microgrid. A safe corollary to that might be, “Unlimited free storage is even better.” Yes, electrical storage batteries are a big part of the future. Hydrogen storage for fuel cells is also likely a few years down the road. The current reality, however, is megawatt capacity batteries remain a financial and technical stretch for most microgrid organizations. For that matter, hydrogen production remains heavily tethered to fossil fuels and long-standing storage limitations.

So back to that unlimited free storage…Planet Earth is a terrific thermal battery with near infinite capacity. With constant soil temperatures unaffected by the seasons just 10 meters below the surface, North American microgrids all share a heat source (or heat sink) with constant temperatures. Deep soil temperatures in the US range from 77°F in Miami, FL, to 42°F in northern Montana, and they remain constant 12 months per year.

Using stored solar heat underground throughout the winter, a GHP consumes roughly 1 kW of electrical energy to pump 4–6 kW of thermal energy from the ground. Conversely in the summer, that same machine pumps heat back into that relatively cool ground where it is quickly absorbed.

Consider how the 52°F deep soil temperature in Iowa compares to aboveground summer temperatures over 100°F and the subzero cold of winter. Clearly, 52°F soil is a far better heat source/sink than the open air. This represents the simple logic of using the earth as a thermal battery.

Financially, the infrastructure cost of renewable energy from an underground thermal grid compares extremely well with photovoltaics and wind generators. Not only is GHP first cost favorable to either wind generation or solar PV, but the cost per renewable kWh is significantly lower with GHPs. Adding in life cycle considerations, geothermal heat pumps are an even more compelling investment for a microgrid network.

6. Climate Resilience is an ever more critical element of long-term energy planning. If a given microgrid has unlimited fossil fuel or 24/7/365 cheap electricity, read no further; good old technology gas furnaces, air source heat pumps, and simple electric heaters are all that’s needed. However, there are changing circumstances to consider. Warmer temperatures are severely testing the limits of air sourced cooling systems. Even today, existing systems in the southwestern US are operating at nearly half capacity when temperatures reach 110°F. The same losses are seen for air source heat pumps at temperatures below 0°F. New government mandates for lower global warming potential refrigerants are likely to make this situation even more challenging.

To be fair, air source heat pump systems may be designed to operate at full capacity under extreme conditions, but the necessary tradeoff becomes extreme losses in efficiency. Because GHPs mine a heat source deep beneath the earth, they are relatively immune to the larger temperature swings of a warmer planet.

Another consideration related to climate resiliency is infrastructure durability. Simply put, all the air conditioners that were submerged or destroyed by Katrina, Sandy, Irene, Harvey and a host of other famous climate events have been replaced at significant cost to homeowners and taxpayers alike. The buried geothermal heat exchangers of coastal America and Tornado Alley remain 100% functional then and now. Any microgrid with exposure to catastrophic seismic, fire, or weather events would benefit from looking below the surface to fulfill that 50% of the energy demand needed for heating, cooling, and hot water.

As distributed energy continues to change the way power is produced and consumed, there are opportunities to do things better and use all of the resources at our disposal. And remember, you may be standing on one of the most valuable renewable sources available.