getting hot and bothered about geothermal
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The Architect has been hot about geothermal and, having a degree in geophysics and hearing about the potential energy savings (up to 50 to 60 percent!), I’m warming up to the idea. However, that geophysics degree cuts both ways, so I’ve not been entirely convinced, especially since we heard the city’s HVAC guy allude to heat buildup issues.
Ground source heat pumps have been around for quite awhile. The first ground source heat pump (coils in trenches) was installed at a home in Indianapolis in 1945. Since that time, more than a million systems have been installed across the United States. Despite that, it’s still considered an innovative technology, primarily because there are not many system installers and, despite the one million installations, not all that common.
First, a word about terminology: Merriam-Webster defines geothermal as “of, relating to, or utilizing the heat of the earth’s interior.” Traditionally, geothermal refers to tapping into the earth’s heat for energy. For example, Iceland is a world leader in green energy due, in large part, to its geothermal resources. Geothermal as used in the context of HVAC systems is actually a ground source heat pump. Yes, heat from the earth is harvested to heat the home (albeit with a heat pump), but the earth is also used to dump heat accrued during the cooling season. Other names for this technology are geothermal heat pumps, earth-coupled heat pumps, earth exchange systems, and GeoExchange systems, the latter a protected trade name. Despite the usage of geothermal in the title of this post, I will use the term “ground source heat pump” because it’s a more technically accurate term than geothermal.
Ground source heat pumps use the earth to increase the efficiency of heat transfer for air conditioning and as a source of heat for heating. Below a certain depth, typically 10 to 20 feet, the earth is at a constant temperature, reflecting an average of the year’s overall temperatures (unless you live near a magma body, in which case you probably shouldn’t be living there!**). For example, the ground temperature in northern Minnesota is about 37 degrees F while the ground temp in Austin, Texas, is about 71 degrees F. Given that the ground temperature is a function of the average air temperature and air temperature is governed in large part by the sun, ground temperature is a form of stored solar energy.
Heat naturally flows from higher temperatures to lower temperatures via conduction. A heat pump induces heat to flow from a lower temperature to a higher temperature, hence the word “pump”. A standard air conditioner is an air source heat pump. It pulls heat from your inside air and transfers it to the outside air. A refrigerator is also an air source heat pump, pumping heat from the fridge interior and transferring it to the exterior (which is why it’s important to keep those coils on the back dust free and breathable). The reason ground source heat pumps are more efficient than air sourced heat pumps is because the temperature difference is greater at the heat sink. Dumping heat into the ground at 71 degrees F consumes a whole lot less energy than dumping heat into the great outdoors at 104 degree F.
There are several types of ground source heat pump systems, including closed loop and open loop. I’m only going to dwell on closed loop systems because (1) that’s what’s most common in Texas and (2) the water conservationist in me is appalled with open loop systems which commonly pump groundwater and then dispose of it. There are also horizontal loop and vertical loop systems. Again, because they are most common in Texas and in urban settings, I will focus on vertical loop systems.
A typical HVAC system, such as the one we have in our current house, has an air handling unit inside the house and a condensing unit located outside. The air-handling unit moves air from the house into the unit and over cool evaporator coils coursing with chilled refrigerant. The heated refrigerant is then moved to the condensing unit where the refrigerant is cooled and condensed (which is why there’s always a fan blowing out there) before it’s moved back into the house to start the whole cycle over again. An HVAC system using a ground source heat pump works in a similar way except that instead of transferring heat to the air via a condenser the heat is transferred to the ground by first transferring the heat to a water/antifreeze solution via a heat exchanger and then pumping that solution through tubing buried in the ground. The tubing forms a loop through which the water is circulated such that the solution is cooled by the time it makes the full loop. The cool thing about this (besides the resulting conditioned air) is that there is no need for an outdoor condensing unit: more room outside and no fan blowing and going all summer!
Heating in a typical HVAC system is accomplished by pulling air into the air handler and across an electric heating element or a gas-heated element. A ground source heat pump works in the opposite direction during the heating season where the coils on the inside are now used as a condenser instead of an evaporator and the earth tubing is used as a source of heat instead of a sink.
The length of the tubing exposed to the earth is an important consideration and is a function of the thermal properties of the ground at your particular location, how big of a system you need (tonnage), ground temperature, groundwater flow, the diameter of your borehole, what you fill the borehole with, and how close your boreholes are to each other (or any other ones in the area).
Distance between boreholes is important to prevent thermal interference between the boreholes. The rule of thumb is 20 to 25 feet between each borehole with about 150 to 300 square feet of land per ton of system capacity. Ideally, borehole spacing should be based on the more specific thermal attributes of your location. The National Ground Water Association says that rules of thumb should not be used when sizing these systems (I include them here as a screening test. For example, if your contractor wants to place your boreholes 5 feet apart, you may have an issue and can start asking questions). There are manual and numerical methods for calculating the length of borehole needed to achieve the desired cooling and heating effect. Hopefully your contractor knows how to run these numbers. Given that a lot of assumptions are made on thermal properties, you'll want to go along with any safety factors built in, even though it may increase the cost. While it's possible to test and quantify the thermal properties at your location, this is typically not done unless your system is expected to require more than 100 tons of cooling.
There needs to be a good thermal connection between the borehole and the earth. Early ground source heat pumps struggled because installers used non-thermally enhanced grout (grout is the stuff that backfills the borehole around the tubing). Thermally enhanced grouts can reduce the depth and number of boreholes. When grouting, it’s critical that the grout doesn’t bridge in the hole creating large air gaps. Air is a terrible heat conductor and will greatly decrease the efficiency of your system. Being a hydrogeologist, I have some experience with drillers. Drillers are infamous for cutting corners on the way to getting the job done, especially if they’ve fallen behind (it’s difficult to check their work when it’s hidden…). Therefore, it’s critical to have a technician babysitting the rig and the roughnecks. Preferably the technician can cuss like a sailor: It was sitting on drill rigs that I learned how to use the f-bomb as a noun, verb, adjective, and adverb, often all in the same sentence.
Drilling is a messy business; therefore, it’s critical to keep the inside of the tubing clear of mud and dirt. According to some experts, lack of cleanliness has caused many a system to fail or serve at a less-than-ideal efficiency.
A shallow water table helps to enhance the thermal connection to the earth because the fill materials will likely become saturated with water. Even enhanced grout typically has lower thermal conductance than the ground itself; therefore, smaller diameter boreholes tend to have better performance (but they increase the chance of bridging…).
High-density polyethylene pipe is typically used for the tubing. High-density polyethylene pipe typically has a 50-year warranty with independent tests suggesting a 200-year lifespan. Pipe joints should be thermally fused (that is, melted together). This material and fusing is what is used for natural gas lines (with a reported 1,000 year expected lifespan). Other methods of joining pipes have been shown to fail over time.
Most installers use geothermal transfer fluid, something the cool kids called GTF, which consists of water and methyl alcohol (to prevent freezing), in the tubes. If the tubes leak for some reason, you won’t have a hazardous waste site on your hands.
Using a ground source heat pump will impact temperature in the earth, albeit locally. Ideally, you live in a place where the amount of heat you put into the ground during the cooling season equals the amount of heat you pull out of the ground during the warming season. Because of unretrievable heat loss around the fringes of your boreholes, this ideal climate would require slightly more cooling than warming. Unfortunately, very few of us live in this ideal climate; therefore, the ground will have a net heat loss or heat gain resulting in a decrease or increase in ground temperature over time. This overall change in ground temperature will decrease system efficiency over time. That’s the bad news. The good news, at least for homeowners, is that this typically only happens with larger commercial systems (many more system tons; therefore, many more boreholes) and not home systems (although it could if the system is not designed properly). Nonetheless, boreholes need to be spaced farther apart in Texas than elsewhere in the country because of our cooling dominated climate.
Design software typically only considers the conduction of heat in the ground. The movement of heat via groundwater flow (advection) is another potential source of heat dissipation; however, water has to be able to flow quite freely for there to be a benefit. Given that our lot is over the Austin Chalk and that groundwater doesn’t move very fast through the chalk (it still has a fraction of its original seawater in it, fer cry eye!), only considering conduction makes conservative sense. The density of the Austin Chalk as well as its lower porosities and saturated condition maximizes its thermal properties.
An addition to a ground source heat pump system that can bleed off some of that heat and also save energy is a device called a “desuperheater” (I picture the little guy on Fantasy Island opening the water heater closet and yipping “De Superheater! De Superheater!”). A desuperheater harvests heat from your central air conditioner’s compressor to heat your water (according to my engineer bride, desuperheaters are also used at power plants). For a ground source heat pump, instead of dumping all that heat into the ground, it gets used to warm your water. Brilliant! A supplemental heat rejector to deal with our cooling-dominated climate! In fact, desuperheaters are highly recommended for cooling-dominated climates (they don’t help you out at all when you’re not using your air conditioning).
A desuperheater can provide about 5 to 8 gallons of hot water per hour per ton of cooling capacity. In an average climate, it can provide 20 to 40 percent of a home’s hot water )and you gotta believe that's higher here in Texas). It provides a greater cost benefit in homes with electric water heating than gas water heating because heating water via electricity tends to be more expensive. Given Austin’s warmer-than-average climate, a desuperheater can reduce water heating energy costs by 85 percent (!!!) if you heat your water with electricity and by 60 percent if you use gas. Given that heating water by natural gas is already 60 percent less expensive than using electricity, the cost saving are considerably less (48 bucks a year for gas versus 152 bucks a year for electricity). However, despite the lower savings, there should be a hidden efficiency benefit for the entire system by using the desuperheater as a pre-earth circulation heat rejector.
If you’re using electricity to supplementally heat your water, you can get a single tank, although some recommend a separate buffer tank for the desuperheater regardless of the energy source. If you are using gas, you’ll need a separate desuperheater tank to preheat the water before it goes into your water tank (haven’t quite figured out why yet…). Kind of a bummer, but there are desuperheater tanktoppers to reduce the footprint of the extra tank. An added plus of a tanktopper is that it pre-preps you for solar water heating (something to consider for the non-cooling months). And yes, you have to have a tank for the desuperheater (the harvested heat has to be stored somewhere); however, none other than the U.S. Department of energy says you can use a desuperheater with a tankless water heater.
Here’s a comparison of annual cost between different types of heating and cooling systems in Austin according to Action Mechanical Systems:
Heating AC Hot water Total
All-electric: $864 $689 $179 $1,732
Natural gas: $418 $689 $75 $1,182
Geothermal: $182 $415 $27 $624
A ground source heat pump with a desuperheater can result in energy bills 64 percent lower than an all-electric system and 47 percent lower than a natural gas system. That’s rather amazing! And according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (I hear their conferences are off the hook…), maintenance costs for ground source heat pumps are 11 percent lower than a natural gas system and 17 percent lower than all electric air source system.
Designing and installing a ground source heat pump requires some special skills and experience. It’s recommended that the contractor be certified by the International Ground Source Heat Pump Association and have considerable installation experience. Engineers and system designers can become Certified GeoExchange Designers via the Association of Energy Designers. Drillers should be certified by the International Ground Source Heat Pump Association. In addition, the State of Texas licenses water well drillers for the drilling of closed-loop geothermal wells.
But what about cost? Unfortunately, up-front costs for these systems can be quite a bit more than a standard system (one source suggested twice as much). In defense, practitioners point customers to the much lower cost to heat and cool after the system is put into service. One source noted that a geothermal system can be a money maker right away if the additional costs are wrapped into a home loan. For example, if financing the additional cost of the geothermal system adds $40 a month to your house payment but you’re saving $70 a month, you’ve just put an extra $40 a month into your pocket. A pretty convincing financial argument assuming you have some headroom on your financing. There also may be financial incentives to take the edge off. Last I checked, Austin (may) offer a $500 to $1,250 rebate depending on the efficiency of the system and whether or not you include a desuperheater. The feds may offer a rebate as well.
Some folks around the corner from us (we met them via the blog) recently finished a Craftsman-style green house (literally and greenily!) with a ground source heat pump, and they were kind enough to give us a tour (they even showed us their desuperheater, which I now have a far greater appreciation for…). They seemed pretty happy with their system.
So, after almost writing a thesis on this topic (sorry...), where does that leave us? I’m just about convinced that we should put one in. Next step is to talk with a ground source heat pump contractor and see what the cost difference is. As you know, it’s all about the green: the green green and the Benjamins…
Sources used for this post:
Retrofitting the Workforce: Geothermal Heat Pumps (focused on Texas)
** As you go deeper the temps gradually increase because of the geothermal gradient caused by a molten hot core bleeding heat to the surface.
Photo from Wikipedia Commons.