Week One: GeoExchange and Thermal Batteries
On a simplistic level, we use energy in its various forms nearly equally to power our buildings, our transportation, and our industrial activities. In our building systems, over half of all the energy use (or approximately one-sixth of the total) provides us thermal comfort, what we generally refer to as heating and air-conditioning. Especially when it comes to heating, shifting away from fossil fuels meets great resistance because of the built-up infrastructure of distribution networks for natural gas and heating oil, the proliferation of equipment to convert the fuels into heat, and the high reliability of these networks of energy transfer. In order to supplant this significant portion of damaging, but most necessary - at least in the case of building heat - energy use, we need a technology that provides an equal level of reliability and efficacy without relying on the use of a limited resource. Thankfully, for over half a century, we have had that technology: geoexchange.
Geoexchange, also known as geothermal* or ground-source, systems work on a couple of basic scientific principles. Before getting into some more detail, I want to introduce geoexchange with an analogy, that of the rechargeable battery. We all understand that a battery holds onto a set amount of electricity for us to use to power some electronic device, and many of us have seen the battery packs that we can take out of a device and plug into a wall to recharge. Our phones and computers have rechargeable batteries, and although over time the constant charging and recharging can wear a battery out, for the normal life of an electronic device, it gets the job done. Geoexchange works on a similar principle, except that instead of energy in the form of electricity, we move energy in the form of heat, and instead of using a chemical solution to store the charge carrying the energy, we use the earth to absorb and store the heat.
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| US Department of Energy |
First, one of the fundamental principles of nature states that thermal energy moves from a hot area to a cold area, but never the reverse. Although we may feel a wave of cold "moving toward us", the movement of thermal energy always moves hot to cold. This is especially important to understand as we think about things like refrigerators, where we do not "add cold", but rather try to take heat away from a cold area and move it to a hotter one. In order to do that - essentially violate a law of nature - we have to add significant amounts of energy (although refrigerators have become much more efficient at moving heat over the last twenty years). Our homes have window air conditioners or equipment that sits on the ground outside the house (called condensing units) that reject heat from our house to the outside. This law of nature, and the methods we use to violate it, play significant roles in the technology.
Second, some materials do a poor job of holding onto heat and get hot very quickly, then cool off equally as quickly. These materials have a low specific heat, which is a measure of how quickly the temperature of a material changes for each unit of thermal energy added to it. For example, in the liquid state, water changes temperature rapidly when heated, and once we remove the source of heat, it reaches the temperature of the surrounding room relatively rapidly. On the other hand, for a material like stone, it takes a significant amount of heat to change the temperature of the material, but once heated, it can give off heat while slowly dropping in temperature.
Lastly, it helps to understand the concept of evaporation. We all have seen steam rising from a boiling kettle, but some liquids do not need that level of heat to evaporate, and in fact will evaporate at room temperature or lower. Some may know of an extreme case of this with dry ice, solid carbon dioxide, which changes from solid to gas at room temperature in a process called sublimation. This process of evaporation at low temperatures holds the key, especially when we talk about heating with geoexchange.
So how does geoexchange allow us to heat and cool a building with a minimum of fossil fuels? The basic operation uses the relatively fixed temperature of the ground (below around four to five feet beneath the surface) of anywhere from 45 to 75 degrees depending on where one lives. In the summer, when we like a space temperature of between 74 and 78 degrees, we can run cool fluid (usually water or a water-antifreeze mixture) from the ground at nearly the temperature of the earth, and either bring it directly in contact with air from the building or use a heat pump to draw heat from the space and add that energy to the fluid. We then return the fluid, using a fluid pump, to the earth. With the added heat, the fluid now has a higher temperature than the earth, and will flow from the pipe to the earth. We help this process by surrounding the pipe with a grout that has high thermal conductivity (meaning it allows heat to quickly pass through it), and since the earth has a high specific heat, we can add significant amounts of heat without changing the temperature. The cycle then starts over with the lower temperature water returning to the building.
In the winter, when we need heat, we cannot rely simply on the temperature difference, since the earth still sits at 45 to 55 (in the areas that need heat, we get to 75 in areas where air temperatures do not drop to the point where we need heat) and we want space temperatures of 68 to 72 degrees. In this case we do need a heat pump, which uses that evaporative property in a material called a refrigerant. The refrigerant in this case will evaporate at the temperature of the earth, pulling heat from the fluid and sending it back down slightly cooler than the ground temperature, then reject that heat into the space to maintain the temperature. This still requires an input of electrical energy, but a significantly lower input than the chemical energy needed to provide conventional heating. The fluid returns to the earth to pick up more heat and the cycle continues.
The geoexchange system has some drawbacks. The system requires that we bury a series of plastic pipes either deep underground (in the case of a vertical system, sometimes as deep as 500 feet) or throughout a large area (a horizontal system, and depending on the building size, this could mean almost an acre). This requires a significant capital investment, although over the life of the system (which lasts a minimum of fifty years before anyone would have to think of digging up the pipes and starting over) that cost requires much less investment than the annual cost of paying for fuel to heat and cool a building. Also, the system still requires a source of electricity, and if that electricity does not come from a reliable, renewable source (more on that in the coming weeks), we have not eliminated all of the potential damage caused by our need for heating and air-conditioning.
The greatest impediments to widespread implementation of geoexchange systems fall under three main headings: logistic, application, and economic. Some buildings do not have the requisite land area or underground access to install the pipes necessary for the system to work. Innovators have begun looking into phase change materials that can provide the thermal battery storage nearly equivalent to the earth, but without the need to dig, but that will take some time to reach the market. Geoexchange systems work best when the amount of heat needed in winter nearly matches the amount available in summer, or when the amount removed or added matches the rate at which the earth can rebalance its temperature. When we cannot achieve that balance, the application of geoexchange requires either a supplemental heat source (further reducing the benefit) or the addition of solar collectors to increase the summer thermal storage (which increases the cost), since over time drawing heat or adding it faster than the earth can tolerate will greatly reduce the performance of the system...even to the point where the building systems will no longer work. Lastly, our energy delivery market currently centers around the selling of electrons and molecules, not the movement of heat. Since we have nothing to sell after the initial installation, investors and financial markets have not yet found a way to value a system that relies on such little input energy. This last challenge faces all green technologies: how do we price something that is ubiquitous and readily available?
Over the coming weeks, I will look at other strategies and technologies that work in concert with each other and geoexchange, forming a palate of solutions from which homeowners, designers, and builders can choose today and in the near future to create a "fuel-less" economy. Some of these, like hyper-insulation, solar photovoltaic, and battery storage, work hand-in-hand with geoexchange, while others, such as earth tubes and energy recovery, can work in place of it. If you hear of a technology about which you would like to learn more, please let me know and I will try to include it in the series.
* Although geothermal has more popularity in the building industry, another more appropriate application of the term geothermal applies to underground hot springs that can be tapped to produce electricity. Because they both relate to the energy industry, geoexchange more appropriately describes the process used in buildings.
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