Geothermal Power has been in the news lately, as many households are turning to this power source to provide their domestic energy requirements. At first blush, this struck me as positive but puzzling: how can a household use geothermal for cooling when it is a heat extraction technology?
Terrestrial heat provides cooling in one of two ways. Large-scale geothermal energy systems convert ground heat to electricity, which powers coolers through normal means. Smaller domestic units implement an HVAC system that uses reinjection to balance and alternate between heating and cooling.
We’ll delve into the paradox of heat cooling by looking at:
- The architecture of geothermal energy plants.
- The cyclical magic of HVAC.
- Prospects for geothermal cooling.
- Rocks on the road.
An understanding of geothermal provides a handle on a major emerging option for managing domestic and business energy requirements.
What Is The Heat Pump Paradox?
It is counterintuitive that a device that works by extracting heat from the earth should be able to cool buildings above the earth.
As a heating solution, geothermal is compelling. This view was evidenced in early applications that involved tapping naturally occurring hot springs. What’s puzzling is how a heat extraction technology can lead to the cooling of a building. The addition of heat would worsen, not alleviate the need for cooling.
We will look at the two ground heat extraction technologies that solve the problem in two very different ways. They differ in scale, topology, and the ways in which they generate cold from heat.
Solution1: Geothermal Energy
We’ll start with the larger-scale solution. The core extraction principle is the same as for the second solution.
What Is Geothermal Energy?
“Geothermal Energy” refers to power plants that use ground heat and water resources to generate energy. These plants require between 300°F and 700°F, which comes from either hot water wells, dry steam wells, or hot rock. These resources are tapped by drilling wells and piping hot liquid to the surface.
How Does Geothermal Energy Work?
Geothermal energy works by extracting heat from below the surface of the earth. The extraction works by passing a liquid that absorbs the heat and returns to the surface.
The core of the earth is a hot place. Magma at the earth’s core and radiation from underground mineral deposits generate heat that increases by 1°F every 70F away from the crust.
Geothermal Power Plants
Geothermal plants come in four basic types:
- Dry Steam Plants: This type of plant is more than one hundred years old, based on an Italian design. The steam that erupts naturally from the earth is trapped, pressured, and directed to turn a turbine.
- Flash Steam Plants: This access hot water deep below the earth’s crust, pressures it, and converts it to steam. The steam is channeled to power a turbine, condensed, and then returned to the ground.
- Binary Cycle Plants: These plants transfer the heat from geothermal hot water into an internal liquid. This second liquid creates steam which drives the turbine. Binary cycle design is a closed system as it does not include groundwater.
- Deep and Enhanced Geothermal Plants: Instead of steam and water, these access heat deep below the earth’s surface. The reliance on a turbine as the end consumer is similar to that of the other system. By virtue of the lack of groundwater, these are closed systems.
The first three types of design are called “Direct Access” plants.
In all variations, the heating principle is the same. The underground temperatures cause an accessible liquid to heat high enough to power a turbine.
How Does Geothermal Energy Solve The Problem?
Geothermal plants have a simple, indirect, and familiar way of achieving cooling. Their turbines generate electricity – much as they would in a conventional coal furnace or nuclear power plant. Consumers of electricity use it to power fans and air conditioners.
The depths required for geothermal energy make it unsuitable for local application. Consequently, the plants feed grids with a wide reach. Local demand for cooling is satisfied by tapping into the grid.
Solution2: Geothermal HVAC
The second solution is a smaller scale version of geothermal heat transfer that does not result in the generation of electricity. Geothermal HVAC is growing in popularity as an air-treatment solution for residential and small commercial sites.
What Is Geothermal HVAC?
Geothermal HVAC (Heating Ventilation And Cooling) is a system of heat transfer between the warm earth and cooler interior of the lived environment. These are small systems that are installed at sites ranging from small communities through industrial sites to individual residential homes.
Geothermal HVAC, being a local solution, attaches equipment to the consumption site so that there is minimal need to connect to the grid. These systems are gaining in popularity on the back of government incentives schemes, offering tax rebates to businesses and householders who install geothermal HVAC.
How Does HVAC Work?
Solar and wind are dependent on (respectively) cloud cover and gusts. In contrast, underground temperatures are stable. The deeper you dig, the warmer it gets. This is due to the insulation of the earth.
Geothermal systems tap into the free reserve of constant heat by protruding a system of polyethylene pipes into the ground.
The pipes convey a liquid (water, refrigerant, or combination) that is pumped into the earth, reaching the depths to which the pipes plumb. Since the pipes are arranged in a cyclical system, the liquid resurfaces indoors.
The resurfaced water is hot, having been warmed by the subterranean earth. This heat is recirculated indoors to create heating. Here the essence of geothermal systems is revealed – heat is not generated through chemical reaction but by transfer.
Components Of A Geothermal HVAC System
A standard HVAC system contains the following parts:
- Heat Pump: This is an integrated unit that directs the flow and indirectly regulates the temperature of the liquid in the earth loop. It includes a condenser filled with circulating refrigerant. Variation in the temperature of the refrigerant triggers the overall air quality regulation of the system.
- Earth Connection: This connects the system to the heat source. Water-filled polyethylene pipes extend into the warm earth below. Their liquid is the conduit for heat transfer between the heat source and the home.
- Distribution System: Ductwork connects the compressor to the atmosphere inside the house. This delivers conditioned air to the home.
Loop systems are ways of arranging the piping as it lodges inside the earth. The earth connection consists of one of four types of loop system:
This is the most common configuration. Typically one of two arrangements is chosen:
- One pipe buried six feet deep, and a second buried four feet under.
- Two side-by-side pipes buried five feet under.
Horizontal looping is cost-effective but requires free diggable land that can be trenched at least four feet deep.
This is a patented variation of the horizontal loop. The pipes are arranged in a spread stacked coil – like a slinky toy. This allows them to contact more surface area of the exposed ground rather than the peripheral contact of the standard horizontal loop.
This configuration cuts down on installation time and costs and makes installations possible in residences that would otherwise have too little available land.
As a variation of horizontal looping, the trench depth is the same. What differs is only the length of the embedded pipe – Slinky uses more pipe.
In this arrangement, holes of four-inch diameter are drilled between one hundred and four hundred feet deep, twenty feet apart from each other.
Each hole houses two pipes running the vertical depth of the hole and joined with a u-bend at the bottom. Grouting improves the performance, limiting heat transfer between the pipes.
The vertically looped pipe connects to a mantle consisting of a shorter, horizontal pipe lain in trenches. The manifold connects the pipes to the heat pump inside the building.
This arrangement is preferred by commercial buildings, schools, and residences where the land area required for a horizontal layout is prohibitive.
They’re preferred where disturbance to the existing landscape needs to be minimized or the soil is not conducive to trenching.
The cheapest option is available to the site that has a water mass (a pond or lake) that meets minimum requirements for depth, volume, and quality. The subterranean pipe is coiled underneath the water body.
To prevent freezing, the coil is placed at least eight feet underneath the water.
The systems above are “closed-loop” systems, so-called because the transfer fluid makes no contact with the environment. In open-loop systems, water from the local environment circulates through the heat conduit. This obviates the need for dedicated transfer fluid in the earth connection.
Surface body water and wells are used as a source. The water returns to its point of origin after cycling through the loop and is replaced at the receiving end with fresh liquid.
Open-loop systems are feasible only where there is a consistent supply of relatively clean water and where local ordinances pertaining to the discharge of groundwater permit.
Geothermal heat pumps use the same mechanism of heat transfer that geothermal energy plants employ. The only difference is the smaller scale. The lower energy requirement permits shallower digging with smaller componentry. So much for warming, but…
How Does HVAC Solve The Cooling Problem?
The solution to the cooling problem is due to the same key that unlocks heat – the Second Law of Thermodynamics. This law provides that in situations of a temperature differential between materials, heat travels from the hotter to the cooler medium until an equilibrium is established between the media.
Essentially, HVAC achieves geothermal cooling by sucking the heat out of your home. The cycle starts with the heat pump increasing the pressure on its refrigerant. Pressure translates to heat. Therefore this heats the refrigerant.
The fiery refrigerant travels through the condenser, coming in contact with the ground loop. Through the Second Law, it transfers heat to the cooler ground loop fluid. This warmed fluid is transferred to the ground, where – again through the Second Law – it transfers heat to the ground below.
The Second Phase
In the second phase of the cycle, the refrigerant is decompressed as it passes through the expansion valve. This has the effect of cooling it. The cooled refrigerant travels through the evaporator coil, where it comes into contact with warm air in your house.
Familiarly, heat transfers from the home atmosphere to the refrigerant, which sets in motion the first phase of the cycle, leading to ambient heat being recycled into the ground.
In this way, compression of refrigerant kicks of a cooling period that reverses the flow of heat between the earth and home in the warmer months, where the heat surplus live above the surface.
Geothermal And HVAC – The Promise
As a cooling source, Geothermal HVAC has a lot going for it:
- Renewability: Geothermal HVAC does not rely on fossil fuels. It does not involve any combustive and catalytic process that releases toxic emissions to the atmosphere. In a world where there is increasing pressure against pollutant technology, geothermal stands to be favored.
- Longevity: Geothermal heat pumps typically have a warranty between twenty-five and fifty years. This reflects the robustness and low maintenance requirement of the equipment. Much of this stems from the simplicity, lack of frictional parts, and the subterranean and indoor enclosure of all the equipment.
- Reliability: There are very little variance in-ground heat temperatures. This makes geothermal much more reliable than solar and wind, which are hostage to the visibility of the sun and strength of the wind.
- Sustainability: Geothermal does not consume a depletable resource. The heat extracted by HVAC in winter returned in the warmer months. Some electricity is required to drive the heat pump, but in energy terms, it is more than offset by the power generated.
- Innovation: In spite of its age, geothermal is not a mature technology. Advances are driving down the cost curve and improving efficiency. These two trends make it applicable in a wider set of contexts. Hot, dry rock (HDR) is the leading source of innovation. Hybrid HVAC systems improve the integration of ground and air systems for thermal extraction.
- Scalability: The technologies discussed here, from slinky loops to enhanced geothermal scale geothermal from national utilities to single dwellings. The reliance on dry ground heat makes it applicable also in contexts that lack natural hot water resources.
- Economics: Geothermal HVAC is highly efficient. One unit of electrical energy delivers four thermal units. Coupled with low maintenance costs, a low fault rate, and investment incentives, the return on investment – compelling to begin with – grows more attractive over time.
- Ergonomics: Geothermal HVAC pumps are much quieter than air pumps. Their ambient hum is like that of a refrigerator. Most of the equipment is buried underground, leaving no unsightly intrusion.
- Support: Governments around the world are encouraging the adoption of geothermal through tax incentives. This reduces the costs and adds to its competitive standing relative to other energy sources.
Geothermal For Cooling – Some Caveats
The geothermal promise is tempered by a few considerations. These, however, relate almost entirely to utility-scale geothermal energy:
- Emissions: Hot rock radiation is a source of dry rock heat. There is concern that geothermal drilling creates escape routes for this radiation, some of which include greenhouse gasses, to enter the atmosphere.
- High Minimum Temperature: Many areas of the earth are not favorable to geothermal mining, as there are prohibitive costs of drilling to the depths where feasible heat reserves are. Favored areas are those that live at the hot edges of tectonic plates.
- Investment Cost: Geothermal involves high upfront investment. This is so especially in the case of power plants. These plants generate a positive return on capital, but this leans heavily on their long lifespans, making the initial investment out-of-reach to many.
- Seismic Risk: There is evidence that geothermal heat mining has the effect of destabilizing rock formation, exacerbating the risk of earthquakes. Data from California has shown a correlation between increased geothermal extraction and seismic shocks.
The only points of concern for geothermal HVAC are the upfront investment costs and disruptive effect of the installation. Both of these are manageable through financing arrangements and patience.