Saturday, January 29, 2011

Residential Geothermal HVAC

A little over a year ago I decided to investigate the possibility of purchasing a new heating system. My oil burning furnace was over 30 years old and the thought of it crapping out in the middle of a cold New Hampshire winter was not pleasant. I started researching my alternatives and quickly started to grow interest in a geothermal HVAC system. Geothermal seems to be something many are interested in but few know about, and the amount of questions I get from those who have learned I've done some research into it prompted me to write this short summary of my personal findings.

A Quick Overview

The basic idea of a geothermal HVAC system is to take advantage of the earth's relatively stable temperature. At a surprisingly shallow depth (~6ft) the earth maintains a moderately consistent temperature year round. The deeper you go, the more stable the temperature is. The actual temperature varies by location, but the ground up my way stays ~49 degrees F. This is much warmer then the average air temperature in the winter and cooler than the average air temperature in the summer, providing a means to more efficient heating and air conditioning.

A typical geothermal system consists of an electric heat pump along with lots of tubing run through the ground to help transfer heat from the earth in the winter (or to the earth in the summer) via a liquid solution which is pumped through the tubing. The larger the home, the more tubing (and land area) required. The way this tubing is run varies depending on where you live. In the good old Granite State, the ground is hard and difficult to push around easily. This is why most systems here are implemented as “vertical” systems. A deep hole or multiple deep holes are drilled straight down into the earth and tubing is spiraled down into and up out of the hole. One advantage to such a system is that by traveling hundreds of feet into the earth, the tubing is able to reach some extremely stable temperatures. A second advantage is that this does not require much property. Vertical systems can typically be installed on small lots.

The other major system type is known as “horizontal.” Here, a bulldozer is used to move some earth and bury some spiraling tube 6-10 feet below the surface. These types of systems are more common in the Mid-West or in areas with softer ground. They tend to be cheaper to install, but require a fairly large plot of land.

A third, though uncommon system type, will lay the tubing in a pond. This requires that you have a large pond on your property, which isn't very likely. If you one of the lucky few, however, this is one of the cheapest ways to implement a geothermal system. Even in a cold winter, the bottom of a large pond won't freeze. It will, however, be colder then the deep earth, so the low cost of installation is countered with lower energy efficiency.

A water/antifreeze liquid solution is pumped around the tubing to an electric powered heat pump, which heats or cools your home. The more heat you can extract from the earth in the winter, or dump into the earth in the summer, the more efficiently the heat pump can run, and the less electricity consumed. These systems are almost always forced hot air systems. They typically can't generate a high enough temperature to work with forced hot water systems, although radiant floor heating is a possibility.

Reliability and Efficiency Factors

The vertical system I was considering comes in two different varieties. Open and closed loop. Closed loop is exactly as it sounds. A small electric pump pumps the liquid solution around a loop of tubing that goes down into the earth and comes back up. The open loop system works like a typical water well (and may even share functionality as the home's well), pumping water up from deep in the ground. The return water is simply returned towards the top of the well where gravity can do its thing.

There is an efficiency curve to these two alternate systems that depends on the home size. Smaller homes are more efficiently heated with a closed loop system because the huge amount of electricity pulled by the well pump (~8 Amps) is avoided. Closed loop systems can operate with very low power pumps that don't have the arduous task of fighting gravity. Large homes are more efficiently heated with open loop systems because such systems generally provide warmer water than can be achieved by recycling liquid continuously around a loop, allowing higher efficiency at the heat pump.

One major drawback to open loop systems, however, is that the well pump is buried in the ground. These pumps can take some abuse (especially if they're running all winter) and will periodically need to be replaced. Of course, it will probably need to be replaced in the middle of the winter, and the cost of replacing a well pump is usually around a few thousand dollars (which should be included in the ultimate efficiency cost of the system). A second drawback is that over a winter season, the greater efficiency offered by an open loop system can start to dwindle as cold water starts to make its way back down to the well. Many times, such systems will include an alternate run off path. Typically controlled by a valve, the alternate runoff allows the home owner to bleed the return water off to a different location for a period of time, allowing fresh warm water to fill the well reservoir. Doing this too long, however, runs the risk of running the well dry.

Having a small home, the closed loop system looked much more attractive to me. In addition to the higher efficiency it would provide for my particular home, I liked the lower maintenance of the closed loop system along with the security of knowing that all moving parts (that could break) would be in my basement and easily (comparatively) repairable. It should be noted that the reliability of the tubing itself is excellent. Your kids' kids' won't have to worry about it decomposing.

Perhaps the most important aspect of a geothermal system is determining the amount of tubing and ground work required for your home. A poor installation can end up being an extremely expensive mistake. If the system is too small, the earth around the tubing will start to cool down in the winter. The heat pulled from the ground will not be able to replenish itself and the system will become effectively useless. At this point, a pure electric heat system typically takes over – Cha Ching! Avoiding this makes a conservatively oversized system sound like a no brainer, but over-sizing the system can reduce running efficiency and can be significantly more expensive to install. Because of these factors, I do not recommend this as a “do it yourself project” and would only recommend going with an established geothermal company with experience and a list of previous customers you can contact. Most of these companies will use a combination of computer software and experience to determine the required groundwork for your house.

The Important Stuff - Money

First and foremost, these systems ain't cheap. My relatively small Cape style home was quoted for a system for just over $20,000. At this time, the government was offering a 30% tax credit towards the cost, but I'm not sure if such a credit is still available.

Most people, myself included, like to think of such huge purchases as investments. So I created a new Excel sheet (actually an OpenOffice Calc sheet) and started attempting to calculate the break even time for my potential geothermal system. Clearly, the upfront cost is significantly higher than other systems, but the energy savings year over year will eventually break even with an alternate oil or natural gas system. After that, the system will start saving money. Easy, or so I thought. The fact is, there are some huge variables here that are almost impossible to calculate or predict.

The first is the actual energy savings. I've found geothermal companies completely unwilling to give me even a gross estimate of the predicted energy costs. The “don't worry I won't sue you if your wrong” line doesn't work here. The best I could get was a list of previous customers with claimed energy savings between 25-75%. That's a huge margin and much of it probably depends on the year. Comparing the savings to a previous year with $5 oil, for example, will show much greater savings than a year with $2.50 oil. As an Electronic Engineer, I know to use the worse case value on a components spec sheet, but using 25% savings greatly increases the length of time required to justify such an investment in comparison to say, 50% or 75%.

Another factor to consider here is the future price of oil / gas. I did some research on the Internet to find that either a) we have tons of oil and if the government just allows us to get it energy will nearly always be cheap or b) we have no oil left and anyone who says otherwise is lying, by 2030 oil will be a bazillion dollars a barrel. I frankly have no idea what the cost of oil or gas will be in the future, but my guess is it will continue to increase either due to supply issues, government mandates and programs or a combination of the two. The rate of this increase can have a large impact on the return on investment of a geothermal system. I also tend to think that going into the future, the cost of electricity will eventually start to fall with new alternate energy developments and this too improves the long term prospect of a geothermal system, though again, this is just a prediction.

One more factor to consider is the value such a system adds to a home's value. Again, there is not much information to go by here. There are so few homes with geothermal systems on the market it makes research difficult. It also seems greatly dependent on the year. On a year when oil / gas is very expensive, buyers may place much more value in a geothermal system then on a year with cheap oil / gas prices. There is also somewhat of an emotional role here, depending on the buyer. The “green” conscious among us may see such a system as very appealing for non-monetary reasons, and may be wiling to pay more because of it.

In the end, an accurate calculation is nearly impossible. I estimated a reasonable expectation of a 10 year break even point, but could see it easily being longer than that. As the home my family and I are currently living in is our starter home, it just didn't seem worthwhile. My wife getting pregnant around this time sealed our decision to hold off. I still think such a system can make sense for many out there however, and hope that for my next home (which I'll most likely live in for a long time) I am able to use such a system. In certain situations, such as when designing your own home from scratch, a geothermal system can be planned in for a much lower cost, such as if it is included as part of the well system. Combined with a solar array to produce electricity, such a system can provide for a nearly complete “of the grid” solution. I like to dream big, but think that realistically this type of setup, or something similar, will be very affordable and even popular within the next few decades.

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