In last week's post, I pointed out that what we call geothermal energy here in New Hampshire should more correctly be called geoexchange. We are not using the energy deep in the earth's crust to generate steam which can be used to generate electricity: instead, we are harnessing the moderate temperatures that lie six or more feet below the surface. Using ground-source heat-pump systems we can harvest this low temperature energy as a heat source to heat our homes in the winter and then, in the summer months, we use the earth as a heat sink by pumping the excess heat in our homes into the ground. It takes energy in the form of electricity to harvest this low grade energy, but with modern heat-pump devices we can draw more energy from the earth than we consume as electricity to run these devices. It is also necessary to remember that the actual source of this energy comes from solar radiation warming the earth's surface, so geoexchange is, in effect, an indirect form of solar energy.
Geoexchange systems are used in a variety of applications, from the heating and cooling of individual homes to large systems that are used to condition multi-building college and hospital campuses. It is a technology that is finally beginning to make headway even though it has been available since the 1940s. Twenty years ago the EPA noted that "Geothermal exchange systems are the most efficient, environmentally clean, and cost effective space conditioning system available." Millions of systems have been installed worldwide, and there are probably between 1000 and 2000 of these units installed in NH alone, most of them in private residences.
A great example of a large and successful geoexchange project is that installed near Boscawen, NH, at the Merrimack County Nursing Home. This is a long-term care facility that houses about 290 residents and it has a staff of 480. It is 235,000 square feet in size and it is a thoroughly modern, well-designed and sophisticated operation providing long term care in a positive caring environment. It is the largest facility of its type in New Hampshire. A picture of the facility is shown below.
The whole building is heated and cooled by a ground-source heat-pump operation. When the design of the facility was undertaken in 2005, there was an early commitment by the design team that the facility would incorporate a geoexchange system to heat and cool the facility. The result was a well-designed building that uses water drawn from 16 standing wells on the property to circulate through 326 individual heat pumps which draw the energy out of the 60oF well water in the winter. The well water, except for a small bleed stream, is returned to the wells at 55oF. This same system is used to provide air conditioning over the hot summer months. There is a heat pump in each of the residents’ rooms, and the residents are able to adjust the temperatures in their rooms to between 68o and 75oF.
During the commissioning stage, the geoexchange implementation team, as on any large-scale project, had to deal with a slew of start-up problems associated with the heat pumps, reliable circulating water pump performance and electrical wiring issues, but over time these were all solved. The system now runs smoothly and maintenance issues are few and far between. The EUI number for the present facility is about 60 kBTU/sq. ft. The US average for equivalent nursing home operations is, according to the 2003 Commercial Building Energy Consumption Survey, 124 kBTU/sq. ft. which means this nursing home has an energy foot print one-half that of its peers. Impressive performance indeed!
I had the opportunity to take a guided tour of their operations with the administrator of the nursing home, Lori Shibinette, and her facility staff. Lori is a recent graduate of Franklin Pierce University's MBA program and she is justifiably pleased at what her team has been able to accomplish with the geoexchange operation. They have no back-up fossil fuel heating system and they are solely dependent on the geoexchange system for heating and cooling and keeping 290 residents comfortable. To ensure reliable 24/7/365 operation, the system incorporates backup pumps and wells. In the case of an interruption of the electrical supply, they do have a diesel fired back-up generator on site to keep the lights on and the geothermal system running. At the start, the maintenance team was a little skeptical of the operation but they are now big fans of geoexchange system. In contrast to most boiler rooms I have been in, the main geothermal exchange control hub was a quiet clean operation with a number of pumps, a large piping manifold, and a computer-monitored control network. A picture of part of the piping manifold is shown below.
Costs for the geothermal operation were approximately 3% of the overall investment for the new building, which is right in line with similar systems, and the calculated payback ranges between 2 and 4.5 years, depending on what assumptions one uses. Regardless, five years after the installation, the extra costs have been recovered and the facility will now for a long, long time benefit from their low EUI and the attendant energy and costs savings. This is clearly a well-conceived and -implemented project that has delivered on its performance.
Let's turn now to a topic that has been on my mind a lot the past few weeks: the installation of a geoexchange system for a stand-alone residence. In this case, the incremental cost of a ground-source heat-pump system is a larger percentage of a new home construction project, making the investment a little more challenging to justify. As a result, homeowners need to put a lot of thought and analysis into their decision whether to install a ground-source heat-pump system or not. That is exactly what I have been doing by reading and chatting to folks about geothermal systems. As fascinated as I am by the technology, I have learned that there are a number of crucial considerations to take into account.
One of the considerations is that here in NH we have a somewhat unbalanced energy load when it comes to conditioning our homes. In my post, A Hundred and Ten in the Shade, I wrote about heating and cooling requirements in NH as indicated by heating degree days (HDDs) and cooling degree days (CDDs) and I noted that in NH we have a heating-dominated energy load as we need much more heating than cooling. In Florida, the situation is exactly opposite. This is shown by the figure below which provides the typical annual HDD and CCD day requirements for New Hampshire and Florida.
These unbalanced winter and summer energy loads can cause problems with geoexchange systems. In the winters here in NH, a geoexchange system is constantly drawing heat out of the ground to compensate for the 6000 HDDs. As a result, ground temperatures around the ground loops drop, and in poorly designed systems the heat pumps can actually draw so much heat from the surrounding ground that the ground below the surface can freeze. In winter, ground temperatures can drop to below 32oF in closed loop systems, which is why these systems usually use antifreeze as the circulating liquid and why they must be designed and sized to operate at very low ground temperatures.
In summer, the reverse happens and heat is drawn out of the building and pumped into the ground and, as a result, the earth around the heat-pump circulating system heats up and can run as high as 75oF for closed-loop systems. The concern with a heating-denominated load is that the heat drawn out of the surrounding area in the winter will not be compensated by solar radiation and the heat pumped into the area during the summer months during the cooling cycle: one could ultimately get a long-term drift to lower ground temperatures which will compromise the operation of the heat pump. A well-designed ground-source heat-pump system takes these factors into account by considering the thermal conductivity of the ground and surrounding rock and ensuring that the ground loop system is provided with sufficient volume to avoid long-term temperature changes in the surrounding rock.In warmer areas, such as Florida, where there is a cooling-dominated load, the reverse can happen. So much heat can be drawn out of the building during the cooling season and pumped into the earth that the ground temperatures rise. On top of this you have surface ground warming from the warmer temperatures all year round and in these systems one can see a long-term drift upwards of the temperatures in the ground which compromises the performance of these ground-source heat-pump units. The figure below shows the temperature fluctuation of a closed loop system over a number of years at Stockton College in New Jersey. This was an early large-scale closed-loop system, installed in the 1990s, that had a cooling-dominated load. Over time, the ground temperatures drifted higher and eventually a cooling tower had to be installed in the geoexchange circuit to relieve some of the heat build-up in the ground.
These ground temperature drift problems tend to happen more with closed-loop systems. In standing well open-loop systems the temperature swings are more moderate, ranging from 45oF in the winter to 65oF in the summer. Open-loop systems rely more on the temperature moderating properties of the water table. Water conducts heat better than earth or granite and slow introduction of fresh water into an open-loop system by movement of the water table and the bleed stream helps to further moderate temperatures. However a good understanding of geologic characteristics and local underground water flow is required beforehand and for these systems a test well is often drilled.
Clearly, as you can see from the discussion above, there are many critical technical considerations involved in the installation of geothermal system. Installing a high performing and reliable system requires proper design and engineering, an experienced geothermal installer and a willingness to avoid "cost savings" shortcuts. Quite frankly, this is not something that you can just rely on your local HVAC contractor to install. Involve a geothermal expert in the design of your system to ensure the equipment you are installing is not undersized or oversized.
Design of a geoexchange system is a task that involves a compromise between the geological characteristics of the area, the heating and cooling loads, the specified equipment and a natural desire to keep installation costs down. Installation costs need to be managed and you need to be very involved but, before you even start, take a look at what you can do to reduce heat loss from your building. Money spent on better insulation and improving the building envelope will help reduce the size and cost of a geoexchange system. Installation costs can then be further reduced by taking advantage of the federal and state incentives. The Federal government provides a tax rebate of 30% for the cost of a geoexchange system and, if you are a customer, PSNH will contribute up to $4500 of the costs through their Energy Star program.
The installation of a ground-source heat-pump system is a complicated endeavor. Be willing to pay for expert advice and project management so that you will end up with a well-designed and -installed system that will, like the Merrimack County Nursing Home project, deliver on its design promises when drawing energy from the cold ground* beneath our feet.
Until next time, remember to turn off the lights when you leave the room.
Franklin Pierce University
(*Cold Ground – This is a tune by a rather obscure group by the name of Rusty Truck which is fronted by Mark Seliger. Mark is better known for his famous photographs of celebrities and musicians including Bob Dylan, the Rolling Stones, Mark Cobain and others. His work has appeared on over 100 Rolling Stone covers. Even though he has been on the other side of the camera lens for most of his career, he clearly has made some friends in the process. On his first album, Broken Promises, he had Sheryl Crow, Jakob Dylan, Willie Nelson and Lenny Kravitz as guests, among others. Enjoy Cold Ground produced by T-Bone Burnett and featuring Sheryl Crow on backup vocals.)
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