Energy System for the Next Millenium
by Dr. John Logan

The Maine Audubon Society's "Visitor Center" in Falmouth, has the most comfortable, cost-effective, environmentally-friendly heating and cooling system available anywhere. This geothermal heating & cooling system has been upgraded in 1999 to use the most modern technology and is a role model in Maine for the preferred energy source in the next millenium.

The major components of this geothermal system are:

• The heat source from a well, depending on reusable heat from the sun, stored in the ground.
• The heat transfer equipment, to move heat to the building in winter and from it in summer.
• The distribution system within the building which performs the heating or cooling.

Economy:
When properly designed, a geothermal system is the lowest cost way of heating (and cooling) because it recycles renewable energy, rather than creating heat by combustion of fossil fuel. The pump in the well, the geothermal transfer equipment, the pumps, circulators, and fans in the distribution system do use electricity. This electricity is the dominant feature controlling the operating cost of a geothermal system. The combination of electricity rates and energy saving strategies, built into the installed geothermal system, determines the ultimate cost of operation. Actual data in Maine shows that residential geothermal heating costs can be as low as half the cost of oil heat and about three times cheaper than propane.

The need for electricity introduces the only source of possible environmental concern for a geothermal system. The geothermal system itself produces zero local pollution.

Environment:
A geothermal system used in a residential building would typically take water from the same well as the water used for domestic purposes. The well, however, has to be correctly sized to make sure that it provides enough heat energy to satisfy the worst-case heating demands of the building. The water, used as the energy source, is taken from the well heat is then extracted from the water to heat the building, and the cooled water is finally returned to the ground in an environmentally approved manner.

The heat stored in the well water comes largely from the sun, which maintains the ground temperature in Maine (below about 30 feet) at about 50°F. For very deep wells, exceeding about 500 feet, there is an added component of heat from the core of the earth, which gives a further heating benefit.

As mentioned above, the single polluting component with a geothermal system comes from the electricity used to run the equipment. In Maine, which has relatively "clean" electricity generation, it can be shown that for the equivalent heating capacity, geothermal is twice as clean as propane and almost four times cleaner than oil.

Selection Considerations:
From both an operating cost and an environmental impact perspective, geothermal heating and cooling is clearly the preferred technology. The question, then, is why geothermal is not more widely deployed in Maine, the state with the highest number of water wells per head of population in the United States?

There appear to be three main answers:

1. Geothermal is more expensive to install, initially, than competing technologies. Geothermal, however, is cheaper to run, has lower maintenance costs, and is cleaner to use than any other source of heating. The key selection consideration is the payback period of how quickly the extra installation costs are recovered from the lower operating and maintenance costs. Typical payback periods are in the range of 2 to 5 years.
2. People are not familiar with geothermal as an energy option. They are not aware that geothermal is available everywhere in the United States, nor do they understand the economic advantage or the environmental benefits.
3. HVAC installers are often not experienced in geothermal systems. This makes both the installer and the homeowner reluctant to try something perceived to be "new". They are more comfortable with the "tried & true". Lack of familiarity, on the part of the installer, may also result in installation cost estimates that are higher than necessary, resulting in rejection of the geothermal option.

Geothermal Heating & Cooling - What it is and How it Works

The Heat Source:
As one goes deeper under the surface of the earth, the temperature comes closer and closer to the average year-round surface temperature. Below about 30 feet in Maine this average temperature is about 50°F and remains at this constant level, all year round, all the way down to about 500 feet below the surface. Below 500 feet, core heat from the center of the earth adds about 3/4°F for every 100 feet of additional depth. For example, a 1500 foot well would have water temperatures at the bottom of about 57°F.

This heat from the sun, stored in the ground, is the energy source used for geothermal heating. Water in a well, of more than 30 feet of depth, will also be at a temperature of 50°F. It is this water, pumped from the well, which is used as the transfer mechanism to move the heat from the ground to the budding. In the case of, what is known as, a "standing column well" system, once the heat has been extracted from the water, it is returned to the well, where it gets warmed up by the ground and again pumped to the building.

To circulate the water through the well, there are 3 main ways to configure the pump and the return pipe in a standing column well, depending on the characteristics of the well:

1. Pump at the bottom of the well and return pipe to the top.
2. Pump at the top of the well and return pipe to the bottom.
3. Pump at the top of the well connected to an induction tube from the bottom and return pipe also to the top.

The pump is connected in the normal way to a pressure tank in the building, from which both domestic water and the water for the geothermal system are drawn. Depending on the different forms of water usage associated with the building, such as irrigation in addition to normal domestic and geothermal use, it may be desirable to use a variable speed well pump to reduce the energy required to pump different amounts of water.

The Heat Transfer Equipment:
The main pieces of equipment, used to concentrate the heat from low grade temperatures of around 50°F to more useful building heating temperatures in the region of 120°F, is known as a heat pump. A household refrigerator is a form of heat pump, which pumps heat from the inside of the refrigerator box to the outside radiation coils, either at the back of the unit or underneath the machine. It might therefore be called an air-to-air heat pump.

For the geothermal heat transfer process, it is convenient to think in terms of different loops of fluid flow. For this discussion, we will consider a geothermal heating system in which the heat distribution to the building is by radiant heat with water pipes embedded in the floor. The heat pump used in such an installation is a water-to-water heat pump, because it transfers heat from one heat exchanger with water at a low temperature to another heat exchanger containing water at a high temperature. The 4 separate fluid loops in such a system are:

1. Water loop from the well pump, through a heat exchanger on the well side of the heat pump, where the water gives up its heat, to the refrigerant and then returns to the well to be re-heated by the ground,
2. Refrigerant loop to transfer the heat from the low temperature well side heat exchanger to the high temperature hydronic (radiant heating) side heat exchanger in the heat pump.
3. Water loop from the hydronic side heat exchanger in the heat pump, where the water takes heat from the refrigerant and flows to the hydronic accumulator tank, in which hot water is stored for the radiant heating system.
4. Water loop from the hydronic accumulator tank through the radiant heating pipes in the floor of the building.

The heat pump is the secret to the energy efficiency of the geothermal heating system since it transfers 4 times the heat energy compared with the electrical energy required to run the heat pump.

For cooling and air conditioning, the heat pump flow process is simply reversed. Rather than transferring heat from the ground to the building, the same heat pump, instead, transfers heat from the building to the ground. The exact same heat transfer system is used, except that, for cooling, the water stored in the "hydronic" accumulator tank is cooled down to temperatures around 40°F. This geothermal cooling process has a very high Energy Efficiency Ratio (EER), because of the low constant temperature of around 50°F in the ground, to which the heat pump is depositing the heat from inside the building.

The Distribution System:
A hydronic system uses heating pipes embedded in the floor to heat the building. This floor piping distribution system cannot be used for cooling because condensation would cause problems with the floor.

Distribution systems which are compatible with both heating and cooling are:

1. Fancoil systems, consisting of a radiating unit through which water flows and over which air is blown by a fan.
2. Air handler systems, similar to fancoils, but installed within a ducting distribution system rather than being stand-alone units like individual fancoils.

In many cases the combination of radiant heating pipes in the floor for heating, together with fancoils and air handlers for additional or supplemental heating in the winter and for air conditioning in the summer, makes an optimal configuration.

Since only one set of equipment is needed for both the winter heating and the summer air conditioning, hardware economies are possible. Building space savings are also achieved because typical geothermal mechanical rooms are half the size of corresponding furnace rooms. Further, aesthetics occur in commercial buildings which are improved by having the equipment indoors, eliminating the need for noisy outdoor condensing units or ugly building top equipment.

Additional Savings:
In addition to providing heat for the building, there is another option available in a heat pump, to heat the domestic hot water. A hot water generator, known as a desuperheater can be added to the heat pump to extract "excess heat" from the compressor. The use of this excess heat from the compressor actually improves the overall efficiency of the heat pump. In effect, the domestic hot water is obtained, on the fly, as a byproduct essentially for "free"!

Other Implementations:
The example used in the above geothermal discussion consisted of a water-to-water heat pump and a hydronic radiant distribution system, with alternative options of fancoils and air handlers. Water-to-Water heat pumps can also be used with baseboard hot water distribution systems.

An alternative heat pump configuration consists of a water-to-air heat pump and forced air distribution. This system typically has lower installation costs than water-to-water hydronic systems, but does not have the comfort level associated with radiant floors. The water-to-air system does, however, automatically provide the distribution system via the ducts for both heating and air conditioning. It is important to note that the hot air provided by a geothermal system has a temperature of up to about 120°F. This is more healthy hot air than the "fried, dried, dust" one sometimes experiences with conventional systems fired by hot air furnaces.

Safety:
The absence of an open flame with a geothermal system provides an additional safety feature from fire risks.