– Combining geothermal heat pumps and technology efficiently

Guadix, in a small village in Andalucia, doesn’t look like much from the distance. You can see a smattering of whitewashed townhouses, a few of the traditional, tree-lines plazas, small shops, nothing too different from any other place in Southern Spain. Once you get a bit closer, however, is not hard to notice a fairly unique corner of town, line with small chimneys in scattered across a few hills, and odd walls and doors apparently leading nowhere. It is the Barrio de las Cuevas, the neighborhood of the caves, the largest concentration of underground homes in Europe with more than 2,000 dwellings.

No one quite knows when people started living in caves around Guadix; it has been inhabited as far as records can reach, and probably long before that. It was a mining town during Roman times; the earliest inhabited caves on record date back around 1,000 years, when Guadix was a trading outpost during the Moorish period.

The cave dwellings are mostly man-made, not natural caves, dug up on the sides of the mountain over the centuries. Andalusia has mild winters, but brutally warm summers. Inside the caves, however, the temperature always remains under 25 °C (41 °F), and cooler the deeper they get. Earth and stone are extremely effective insulators, so the underground houses are a clever response to the extreme Andalusian heat.

The issue is, however than caverns and underground dwellings are hard to replicate in many places. Natural caverns are few and far between, and man-made ones are rather impractical in flat land, and too expensive to build on a large scale.

This does not mean, however, that the principles that make a cave-home so energy efficient cannot be leveraged to cool down larger buildings. Just a few feet below ground, soil temperature remains stable at 16 °C (60 °F). Cellars, man-made underground passages or rooms retain similarly cool temperatures. With clever engineering, cool underground spaces and soil can be used as a form of heat sinks or exchangers, allowing building cooling systems to dump excess heat to earth itself. Geothermal heat pumps rely on this principle, using loops of coolant water underground to dump excess heat using very little energy.

Stable underground temperatures can also be used in cold climates in the opposite direction. When temperatures outside are below freezing, soil temperatures remain around 16 °C (60 °F), allowing a heat pump to work in the opposite direction, using ground heat to warm the building where they are housed.

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Combining earth and technology efficiently

Although efficient, geothermal heat pumps are far from a complete solution. Because they rely on a small temperature gradient, the amount of heat or cooling they can offer is sometimes quite limited in small systems. They might be enough to cool a house from 35 °C (95 °F) to 27 °C (80 °F) or warm a few rooms from freezing to 15 °C (59 °F), but not to offer full climate control. As a result, engineers often must pair this technology with other equipment to offer a complete solution.

Modern energy-efficient buildings rely on a combination of several technologies to provide efficient cooling and heating. The first step is good insulation, as it is much easier to keep a building warm or cool if there is little excess heat or cooling wasted to the outside of the facility. The second step might be using a variety of “natural” solutions to limit the amount of cooling or heating needed. Although a geothermal system might not be enough to lower the temperature from 35 °C (95 °F) to 22 °C (71 °F) on a warm summer day, it might be able to lower the temperature a step or two to 30 °C (86 °F) or 28 °C (82 °F). From that point, modern, state of the art energy-efficient solutions can take over, providing precise zonal control and temperature settings with much less effort than if they were doing all the work by themselves.

Hitachi Cooling & Heating has considerable experience providing this kind of comprehensive solutions. In the Nippondaira Hotel in the Shizuoka prefecture in Japan, the cooling system combines two distinct, complementary technologies to offer climate control with a low carbon footprint. The Hitachi Air Cooled Module Inverter Units are the best of their class, and they are in a class of their own in terms of energy efficiency.

These units collect the air they use from a 180-meter-long geothermal tunnel system built below the hotel grounds that acts like a natural cave. The air in this tunnel system is always around 16-18 °C (60-64 °F) year-round. No matter how efficient a heat pump is, it is easier and cheaper to use cool air to feed an air conditioning trying to beat back 30 °C (86 °F) weather than to try to cool down 30 °C (86 °F) air.  Advanced machinery can be made even more efficient and cheaper to run by pairing it with a well-designed, intentional use of the terrain and soil.

Of course, building a 180-meter artificial cave is not always an option, and it is far from cheap. Geothermal heat pump installations are often expensive, as well and they are quite site-dependent; not all soils favor installing a cooling loop or might require a lot of space to be effective. As it is often the case, good engineering, combined with smart climate control systems that combine several solutions and tools are often the most practical and cost-effective in the long run.

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