Geothermal – The Most Overlooked Alternative Energy

The human population is rapidly increasing on a planet that is finite in size and has a limited amount of non-renewable resources. Our energy use continues to increase, with the heating and cooling of buildings as the single largest source of energy consumption in the United States (nearly 40%). Through our demand for cheap energy, non-renewables such as fossil fuels (oil, natural gas, coal) are being consumed at a rate much faster than they are naturally replenished. There has been much speculation about the inevitable concept of 'peak oil', but the ultimate timing of this event depends on a number of factors. These include the rate of fossil fuel use in the future and advancements in efficiency and alternative energy technology.

Regardless of how many centuries or millennia of fossil fuel might remain buried in the earth, it will never be practical to extract all of it. The economic expense and environmental repercussions of removing and burning these reserves make the swift transition to alternative energies a global necessity. In considering alternatives, a diverse set of investments is the safest bet. Technologies like solar, wind, hydroelectric, and nuclear tend to get the most attention and have proven their worth, but they each have their disadvantages and are only part of the solution.

The Alternative Alternative

There is another alternative energy technology that is often overlooked and misunderstood by the public. “Geothermal” has traditionally been used to refer to natural heat sources near the surface of the earth, ie. any area with volcanoes, geysers, or hot springs. While features like these indicate immense sources of energy, they are geographically uncommon and are often located away from major cities. To reach the source can require drilling very deep and expensive wells, and the extreme temperatures and pressures of these geothermal features are difficult to work with.

How is this a viable solution?

It's not. But... There is another kind of geothermal that deserves some serious consideration. It is referred to as 'shallow geothermal' because it does not require the deep heat sources of traditional geothermal methods. It works on the principle of heat exchange and can be implemented in almost any location. The setup generally uses a heat pump to circulate heat exchange fluid through vertical or horizontally buried PEX tubing.

How does heat exchange work?

Ground-source heat pump systems facilitate the seasonal relocation of heat by pumping fluid through a buried heat exchanger. In the warm months of summer, heat is removed from buildings and stored underground, replaced by fluid chilled by the relatively cooler temperature of the subsurface. In the winter, the heat that was stored underground during the summer can be extracted and used to warm the building.

Why aren't more people doing this?

Shallow geothermal heat exchangers have been around for several decades and are increasingly utilized by people around the world. However, this technology has often required a larger initial expense and has shown a slower payback rate than other alternative energy technologies (solar, wind). The excessive number of vertical boreholes or of horizontal trench length required to effectively control the climate of a large house or building can be cost- or size-prohibitive. To complicate matters, the contractors who install the exchangers may not be experts in geothermal technology and may fail to properly optimize the system.

How can we optimize the systems?

The best way to improve shallow geothermal systems is to increase the thermal conductivity of the exchanger and surrounding substrate. Research studies[1] have indicated that in these systems heat transfer is enhanced when the bulk density, quartz content, and water content of the substrate is increased.

Mineral Conductivity

Quartz is much more effective at conducting heat towards or away from an exchanger than clay or other common minerals. When a quartz-rich substrate is densely compacted around the heat exchanger, this improves the heat exchange connection and further increases the overall thermal conductivity.

Pore Space

Because water is far superior to air at conducting heat, it is critical that the remaining pore spaces remain saturated. Installing the heat exchanger in an aquifer or wet area that is usually below the water table greatly enhances the efficiency of heat transport in the system.

Heat Exchanger Design

The material and design configuration of heat exchangers can be greatly improved over traditional methods. Heat exchangers made of metals like copper and steel have a much higher (800x) thermal conductivity over PEX tubing and can be coiled more compactly to maximize efficiency. This exchanger then requires shorter trench lengths and fewer wells, reducing the largest installation expenses and increasing the payback rate.

Heat Exchanger Installation

A final consideration to further optimize these systems is to take advantage of the flow of water. In some circumstances such as confined aquifers or steep hydraulic gradients, this flow can be quite considerable. If the flow is exploited, the efficiency of the system can be improved as heat will be more rapidly moved toward or away from the exchanger.

In Conclusion

Like the other forms of alternative energy, geothermal has limitations and challenges to overcome. Ultimately the entire suite of alternative energy technologies will need to be utilized to supplement the shortcomings of each method. Geothermal will undoubtedly play a critical role in this transition and help to reduce the overall energy demand by greatly diminishing the power required to heat and cool buildings. As we begin to optimize system design, the technology will become more affordable and widely accepted!

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About the author

I have contemplated and published on geothermal energy for over a decade. Some of the information included in this article is from an unreleased research study, and some of the thoughts discussed are based on my personal ideas.

See internet sources and additional reading as embedded links in the article. All photos and diagrams are in the Creative Commons or by the author

External Sources
_{[1] Dowling, C. B., Neumann, K., & Florea, L. (Eds.). (2016). Geothermal Energy: An Important Resource (Vol. 519). Geological Society of America.}