12. Household Geothermal Energy

Geothermal

13.0 FROM THE GEOTHERMAL EXPERTS:

Imagine all the solar energy that strikes the Earth every day. It’s all stored in the top ten feet of the soil (the temperature below that is constant). Every three seconds, enough sunlight reaches our planet to power the entire world for a year.

Ground and Water Source Heat Pumps (GSHP and WSHP) use the refrigerant cycle to extract all that thermal sun-energy from the ground. The atmosphere is the “heat sink” for regular heat pumps. The ground is the “heat sink” for GSHPs.

The heat sink for “water-source systems” is an open body of water: a lake, river, pond, or the ocean. In an “open source” system, the lake water is itself drawn into the unit. In a “closed source” system, the thermal connection is via a large “field” of sealed plastic piping buried in the lake floor. Instead of transferring heat to and from the air, the GSHP transfers heat to and from the lake (which itself is a giant solar collector for sunlight).

A W/GSHP can provide heating and cooling for a regular forced-air system or for a hydronic radiant heating system (radiators or radiant floors). A W/GSHP can also provide domestic hot water. It’s not hot enough for the tap, but it’s a lot warmer than the groundwater going into the water heater. (If you add a “desuperheater” to the system, the temperature can be increased to 120° F, high enough for the hot water storage tank.)

The heat sink for a “ground loop” GSHP system can be (1) pipes buried in a narrow ditch excavated by a “Ditch Witch,” (2) “slinky” piping buried in a wide backhoe-sized trench, (3) or a vertical U-loop grouted in wells drilled by a water-well rig.

Ground loops can be buried under driveways or parking lots for optimal utilization of available space.

Vertical wells are back-filled using thermally enhanced grout with bentonite for maximum thermal connectivity with the soil. (Many wells are back-filled with a special concrete mix.)

The entire “ground field” of the GSHP system adds that much thermal mass to the building. (Very helpful.)

Two air handlers on the same W/GSHP piping loop can heat and cool simultaneously, in essence, transferring heat directly from one area to another.

There is no equipment outside the house (as with condensing units for airsource HVAC systems), which can help prevent vandalism or weather damage.

W/GSHP systems have superior dehumidification in the summer and warmer air supply temperatures in the winter than air-source heat pumps. There’s no defrost cycle (which saves electricity) to remove ice build-up from the outside coils during the heating mode.

Although GSHP systems are unfamiliar technology to most people:

  • GSHP systems have been in commercial use since the 1940s.
  • There are almost two million GSHP systems installed worldwide. Over fifty thousand new systems are installed in the United States every year.
  • Installation costs are twice that for air-source systems, but the life-cycle cost is much lower because manufacturers guarantee the ground loop for twenty-five years, with an expected lifetime of fifty to two hundred years.
  • The EPA has classified GSHPs as the most energy-efficient, environmentally clean, air- and space-conditioning system. (They’re even better than solar photovoltaic panels, which add toxins when disposed of in a landfill.)
  • Payback for GSHP systems is helped considerably by federal, state, and utility rebates.
  • The payback for GSHP installations in the United States for replacing electric heating is four years, five years for natural gas, and twelve years for heating oil.
  • GSHPs have a much lower refrigerant charge of ozone-depleting refrigerants (which also have an ultra-high Global Warming Potential) than conventional heat pump systems.
  • GSHP heating efficiency is 50 percent to 70 percent higher than other sources, and cooling efficiency is 20 percent to 40 percent better.
  • GSHP systems have a peak efficiency of 300 percent to 600 percent on the coldest days versus 100 percent to 150 percent for air-source heat pump systems.
  • GSHP systems are the best possible heating solution in frigid climates because: (1) everything is either underground or inside, (2) no matter how cold it gets (down to below 0° F), the efficiency never drops below 300 percent, and (3) they require very little electricity to operate.
  • Several large, commercial test installations of solar-geothermal hybrid systems in Alaska with photovoltaic panels have provided space heating more efficiently than any other source.

13.1 GEOTHERMAL POWER IN YOUR BACK YARD.

At twenty feet of depth, the ground temperature throughout America is 62° F all year long, at all latitudes. At fifteen feet of depth, the ground temperature varies 10° F from that norm in a smooth, seasonal (sinusoidal) curve.

The top fifteen feet vary another 10° F seasonally.

These values are everywhere cooler than ambient in the summer and warmer than ambient in the winter. Hence the thermal advantage of using the ground as the heat sink instead of the air, per conventional air-source heat pumps.

Note: For graphics and a topographic map of ground temperatures nationwide, visit the “Build Solar” web page: https://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm

13.2 SWIMMING POOLS AS A PASSIVE SOLAR COLLECTOR.

An in-ground swimming pool absorbs 75 percent to 85 percent of the incoming solar. Add a light-blue swimming pool cover, and you can extend the swimming season a few weeks on either side of summer. A semi-transparent blue cover allows the sun’s high-energy UV radiation to penetrate and be absorbed into the pool.

Why not a black pool cover? Because a black cover blocks the bandwidth of “Ultra-Violet” sunlight that carries the most thermal energy.

The average black solar water panel only absorbs 20 percent of the sun’s energy. Why so little relative to a swimming pool? Because refraction within the water absorbs all wavelengths of sunlight.

 13.3 POLAR BEAR SKIN: THE ULTIMATE SOLAR COLLECTOR.

When you live in the Arctic, you need all the heat you can get. Polar bears may have white fur, but it’s translucent (the hairs are hollow). Their skin is black. The white hair allows polar bears blend in with the snowy environment. The black skin maximizes the warmth they can get from sunlight.

Who’s going to invent a “polar bear pool cover” that looks blue but has enough internal, reflective flecks of reflectance (trapping radiation inside the water) so that the pool absorbs 100 percent of the incoming sunlight?

Likewise, a solar water panel that’s more than 20 percent efficient with adequate thermal mass to absorb more of the sun’s thermal energy before it’s re-radiated, back into the atmosphere.

 13.4 HEATING SWIMMING POOL WATER PRESENTS UNIQUE  CHALLENGES.

If you’ve ever had a swimming pool in the back yard, you may have noticed the water level drops quite a lot every few days. That water loss is via evaporation. If you recall the kitchen lessons, evaporating water requires a tremendous amount of energy, the “latent heat of vaporization.”

When water evaporates from a pool, it draws 540 calories/gram out of the pool. Even a slight breeze accelerates the evaporation process, and an unsettled surface even more so (swimmers, the pool filter running, waves caused by a stiff wind).

The average pool loses 4° or 5° F overnight (60 percent via evaporation, 25 percent to radiation, and 15 percent by convection). So, before you can even think about heating the water to extend the swimming season, you first need to stop that evaporation.

Cover the pool! An inexpensive solar blanket has air bubbles embedded in the plastic, for a little extra insulation.

13.5 SWIMMING POOL HEATERS HAVE THEIR LIMITATIONS.

Up to 70 percent of the heat lost from swimming pools is through evaporation. (If the new water rules in California limit homeowners to barely enough water to bathe and wash clothes with – imagine all those backyard pools! Zoom- zoom, there goes your entire water allowance.)

The rest of the heat is lost through radiation and convection to the ground and air. The ground temperature is relatively stable, roughly equal to the nighttime lows. Outside of the summer months, there are a few months where sunlight warms the ground enough during the day to make an auxiliary heat source feasible.

The only credible source (other than a pool cover) is solar water-heating panels, simple black plastic panels integrated with small water tubes. The black plastic absorbs sunlight, heats up, and this heat is transferred to the flowing capillaries of water. Very simple, very effective.

13.6 A POOL SOLAR WATER HEATER USES ZERO ELECTRICITY.

The pros recommend operating the pool pump at least twleve hours each day. Filter intakes are at the surface of the pool inside the skimmers. And so, when the pump runs, the surface water is always moving.

Unsettled surface? Evaporation!

Since the pump runs anyway, and if the pump can handle the extra static pressure, you can set up pool-heater controls to divert a part of that water through the solar panels.

In the heat of the summer, when the pool water is uncomfortably hot, the solar panels can radiate heat outward at night to cool down the water.

Either way, the solar add-on doesn’t use any more electricity than the regular operation of the filter pump.

13.7 DID I READ SWIMMING POOL PUMP MOTOR? ELECTRICITY  – SAVINGS POTENTIAL!

The old standard for pool pumps was a single-speed pump (running twelve to twenty-four hours a day). Constant-speed pumps are inexpensive but can use more electricity than any other appliance in the house (including the AC).

Some years ago, two-speed pumps gained traction. The up-front cost is more, but the low speed uses exponentially less energy than the high speed. The idea is to use the low-speed for filtration and the high speed when you’re vacuuming the pool or when leaves are falling (the skimmer inlets need extra force to pull leaves from the surface of the water).

Adding solar panels into the circulating loop requires more static pressure from the pump, and volume – hence the higher speed. That high-speed adds to the operating cost for the solar panels.

Variable speed pumps to the rescue: they operate at a full range of speeds. Run the pump very slow for regular filtering (slow enough to have almost no movement of the water at the surface, minimizing evaporation), a little faster when the solar panels are in the loop, and faster still for vacuuming and fall-filtering leaves off the surface.

Variable speed motors are on the expensive side, but if used correctly, they have a decent payback – especially when coupled with a solar panel water-heating system.

13.8 POOL WATER HEATING WITH JUST THE COVER – AND A  LITTLE HELP.

With a baby blue pool cover on, the water is calm. The upper water layer heats up. That warm water remains at the top and gets warmer and warmer, but at a slower and slower rate. Now:

If you run the pool pump at a very low speed, it draws the warm water off the surface, into the skimmers, then dumps that warm water deep into the pool at the discharge jets, mixing the whole volume from top to bottom. This draws cold water up to the surface layer of the pool, maximizing heat transfer from the surface, hence getting the absolute most out of the pool cover.

With a variable speed pump, temperature controls cycle the pump faster when the upper layer is warm, slower when the water is colder – to max out the heat transfer, and so forth. (Aren’t controls fun?)

13.9 SOLAR WATER HEATER ASSIST FOR THE POOL AND HOUSE.

Let’s say you live in a house that has a pool in the back yard. You have a large family– what kids don’t love to swim!–and, therefore, high hot-water demand in the household.

It’s not hard to justify a solar water heating system on the roof. Inevitably it has to be a hybrid system (integrated with a gas or electric backup water heater) to be sure to meet your hot water needs all year long, but solar water heating still has a decent payback in almost any climate.

The days are longer in the summer, the sunlight more direct, and the solar water output of the panels is maximum. Why not size the system a little bit bigger to add a couple of months to the swimming season?

Then, once the swimming pool is closed for the year, you have that extra hotwater capacity for the house.

13.10 WHAT, YOU CAN’T AFFORD A SWIMMING POOL?

Ground Source Heat Pumps (GSHP) are an old technology, entirely dependable, and very efficient. Regular heat pumps use the refrigerant cycle to transfer heat from your house to the air in the summer (vice-versa in the winter). GSHPs use the same refrigerant cycle to transfer heat to the ground via water circulated through pipes buried in the ground.

Why not use the swimming pool as your “heat sink?” With a baby blue pool cover, almost all of the sun’s heat is absorbed, even in the winter. The heat extracted from the house during the cooling season heats the pool water in late spring and early fall. Evaporative cooling from the pool surface dissipates excess heat in the swimming season, precisely like cooling towers for large buildings.

Why spend $15,000 for the ground-loop field on a “closed-loop” GSHP when you can bury those water-circulation pipes into the sides and bottom of a swimming pool when it’s built?

 13.11 USING A SWIMMING POOL AS A SOLAR COLLECTOR.

Consider a GSHP using the swimming pool as a “heat sink.” The capacity in summer to dissipate heat from a swimming pool is very high. So, the limiting capacity of the system is the availability of heat in the winter.

A normal GSHP has a field (buried four to six feet below the surface) laced with piping equal to about 1,000 square feet in area per ton of cooling. The surface area inside a forty-by-thirty-foot pool, average five feet deep equals 1,500 square feet. The pool, however, has the advantage of active convection: cold water at the surface falls to the bottom (the ground is always warmer than the outside air), amplified by the strategic operation of the filter pump.

Moreover, a baby blue pool cover absorbs much more energy than the ground, and that energy is much more accessible.

If your winter weather is severe, the regular GSHP works fine in sub-zero temperatures, whereas a swimming pool can’t ever be allowed to freeze.

So this hypothetical system will work in any location with a mild winter. (Or just run the circulation pump at an ultra-low speed to keep water moving when temperatures drop below 32° F, or 28° F for a saltwater pool.)

 13.12 THE MANY FACES OF SOLAR ENERGY.

This discussion has ranged over the gamut, using solar energy to heat water. The swimming pool itself is a solar collector, absorbing sunlight all year long at 85 percent capacity. The rooftop panels for heating water use sunlight to heat the black plastic panels, which, in turn, transfer heat to the water (at 20 percent efficiency).

The black solar panel acts as a “heat transfer” medium. The swimming pool GSHP model uses sunlight to heat the water directly, to then be transferred to the building loop via a refrigerant cycle: the water-to-water heat exchanger.

All three applications capture solar energy as heat and use it on the spot, in a straightforward and dependable process. Finally, solar photovoltaic panels use solar energy to create electricity, which then is used to operate an electric water heater.

 13.13 THE SWIMMING POOL AS “THERMAL MASS.”

We have seen passive solar heating applied in reference to daylighting: heat a concrete floor or wall via direct sunlight during the day, then use the heat from that wall, radiating it into a home to keep the whole space warm overnight. (The classic southwest adobe-house principle.)

The GSHP swimming pool does the same thing: sunlight absorbed by the baby blue cover heats the water during the day. Heat is extracted by the GSHP system to warm the house overnight.

Entire houses can be heated by a single, thick wall warmed up during the day. Imagine the heat-storage capacity of a 2,500 cubic foot pool versus a one hundredcubic-foot wall. (The “thermal mass” of the swimming pool includes a foot or two of ground on all sides.)

Some passive solar applications gather the sun’s energy in a wall made of water containers because water can absorb four times more heat than concrete. Hence, the swimming pool can absorb one hundred-plus times as much energy as the concrete wall.

The swimming pool’s heat is much more accessible, too, because water moves, physically: warm water rises and cold water falls, creating a steady convection circulation. The same circulation happens in concrete (and in the ground), but on a molecular level, much slower and far less efficiently.

 13.14 SOLAR PANELS WITH A GEOTHERMAL SYSTEM.

GSHPs are capable of extracting (or depositing) heat almost anywhere. For example, in an area with mostly a heating load (e.g., northern states with arid climates), you can bury the piping under a black asphalt driveway. The black tarmac absorbs far more heat than grassy or tree-covered ground, and all that thermal energy can be accessed by the GSHP system.

Geothermal heat pumps maintain a constant Coefficient of Performance (COP) of 4.0 down to 0° F and even lower. The ground pipes can be buried in a lake or pond, and continue working even if the surface of the lake freezes over. (The COP of electric strip heat is 1.0)

Inexpensive solar panels on the roof can also serve as this heat source for a GSHP. (In which case it would be called a “Water Source Heat Pump.”) The system can even be configured as a water-to-water heat pump, transferring thermal energy to and from two water loops: one loop providing space heating and cooling, and the other water loop carrying water from a swimming pool, pond, water well— even seawater for a ship at sea.

13.15 ONCE YOU HAVE HOT WATER, THERE ARE MANY POSSIBLE USES.

The critical thing to remember about Water Source Heat Pump (WSHP) systems is that they’re very good at exchanging thermal energy with any “heat sink.” A concrete wall or deck or even roof receiving direct sunlight all day (with pipes embedded in the roof assembly) can provide passive heating and cooling for a house. Once the WSHP has access to this “heat sink,” the sky’s the limit on how you can use that water.

You can use a WSHP system to provide hot water for domestic use, heat a swimming pool, or divert the hot water into a hydronic heating system of radiators.

Radiant floors are another option. Modern radiant floors are as easy to install as hardwood flooring, with all the piping embedded in the square flooring sections that glue and snap together.

 13.16 SOLAR HOT WATER AS SUPPLEMENTAL HEATING. 

Our hot water application used solar panels as a second energy source to an electric water heater to ensure hot water is available at all times. Space heating uses of solar heat also require a backup heating source such as a small natural gas furnace or, if the backup is needed infrequently, electric (which is cheaper to install but three times more expensive than natural gas to operate).

Nor do you need to solar-heat an entire house: you can install radiant flooring, for example, in just the bathrooms or kitchen, or in the main living area, and use a WSHP air handler to provide space heating otherwise. Just like you can adjust radiators individually, the piping can have shutoff valves to turn the radiant floor on and off in a specific room or area.

 13.17 THE GROUND LOOP AS A GIANT, DYNAMIC THERMAL  RESERVOIR.

Imagine a GSHP with a ground field: piping buried six feet below the surface in trenches or vertical wells. That piping conducts heat to the surrounding soil, and the entire field acts like a giant thermal battery.

In the summer, the GSHP system transfers heat to the ground, where it’s stored. The ground loop field heats up above the average temperature and stays like that because heat is added faster than it can be conducted away into the surrounding earth.

In the winter, you take that stored heat out of the ground. By the end of the winter, the temperature of the whole loop field is now colder than the ambient ground temperature. Consequently, when the system calls for cooling in late spring, it transfers heat from the house back into the ground.

The cycle continues, season by season. So, instead of the daily cycle of a passive solar wall, you have an annual cycle of a passive solar ground.

Design note: what makes the ground loop work is a good “thermal connection” to the ground. GSHP wells have to be grouted carefully, and ground loops, too. The more moisture in the soil, the better the heat transfer to and from the ground loop. As such, if the ground gets very dry in the summer, there may be reduced thermal capacity.

 13.18 USING A GSHP TO MOVE HEAT AROUND TO AND FROM  MULTIPLE LOCATIONS.

I live in the southern Appalachians, in a heavily forested area. The climate is moderately warm in the summer, but the winters are long and cold. I’d love to have an indoor pool for swimming. Given heavy cloud cover much of the summer and deep shading from the trees and mountains (the area is called “Nantahala” by the Cherokee, meaning “land of the noonday sun.”), this hypothetical pool needs heating even in the summertime.

Imagine a GSHP system that has the ground loop embedded in the bottom and sides of an in-ground swimming pool. The air handler provides heating and cooling to my house up the hill. The swimming pool building also needs cooling, and a great deal of dehumidification (remember all that evaporation from pools?)

In the summer, the system transfers heat from the house and the pool building to heat the pool. There’s much more heat to get rid of than the pool needs, so you also have a standard GSHP loop field or vertical wells. That excess heat elevates the temperature of the ground field above the ambient (acting as a thermal storage battery).

Then, in the winter, you extract the heat from the ground to heat the house and, with any excess thermal energy, heat the pool. The WSHP air handler in the swimming pool can dehumidify in the winter (adding all that heat to the water loop), while the WSHP air handler in the house is heating.

Every process at a COP = 4.0.

Or, I run the ground piping across a neighbor’s lot and put that ground loop in Santeetlah Lake, for unlimited heat all year round, enough to heat the water, radiant floors on an outside porch, you name it! Access to the lake makes the pool swimmable all year long, and the total utilities cost would be less than I pay now. Wow!

To repeat: If your indoor swimming pool needs both dehumidification of the building air and heating of the pool, a WSHP system can do both. Dehumidification requires cooling, and the heat extracted from the air goes directly into the pool—from the same water loop!

13.19 CENTRAL HOT WATER HEATING LOOP.

Any combination of these solar heater ideas has hot water flowing to the water heater storage tank in a more or less steady stream. What you end up with is similar to the water distribution system in large buildings like hospitals: a single, main water loop with a circulating pump.

The water in that loop is always hot. And when somebody turns on a fixture anywhere in the building, hot water is available very quickly from a short branch line off the main circulation loop. In this scenario, the water loop is literally an extension of the storage tank.  That loop piping will have “standby losses” and must be insulated accordingly. However, whenever you open a hot water tap, you get hot water right away and not after four to five gallons go down the drain. Perhaps you save enough water to offset evaporative losses from our imaginary swimming pool.

Future Tech! 

13.20

Solar water heating doesn’t get nearly the PR it deserves. It’s cheap, dependable, and has a much faster payback than photovoltaics.

 13.21

Rural homesteads dig up the ground for a septic field, which is just the size and depth needed for a GSHP ground-loop field. The soil is moist all the time, too, maximizing the efficiency of heat transferred to and from the loop piping. Since the ground-loop field is the most expensive part of the GSHP system, using the septic field excavation saves enough money to make the installation comparable with the cost of an air-source heat pump.

13.22

Swimming pool design should be standardized on two-speed filter pumps, with the low speed sized for basic filtration and the high speed for vacuuming.

13.23

Water-scarce areas like California need an ordinance to require pool covers to eliminate evaporative losses. Covers, in turn, will make the water too warm for swimming, so now you need solar panels to radiate heat away at night—solar water panels, which can now provide hot water for the household.

13.24

New residential communities bury water mains under new roads, with one service blanked off at each lot. Why not add ground-loop piping under the road, too, with a tap at each lot for future GSHP heating and cooling?

13.25

Cruise ships can get free heating and cooling (and hot water) from WSHP systems.

All ships—maritime, military, submarines!

13.26

All the new fracking technology to inject water into the ground to extract hydrocarbons. Why not inject all the excess “thermal waste” from power plants into the ground, to then be accessible via GSHP loops for individual and commercial space heating needs.

13.27

Integrated concrete-frame residential construction methods (with solid foam insulation sandwiched between concrete, for walls) has concrete roofs: an ideal location for GSHP “slinky” piping to absorb thermal energy before it gets into the house. A black cover on the roof in the winter maximizes heat absorbed from the sun, for space heating. All the same principles apply as for radiant floor (ceiling) heating and cooling.

13.28

150° F attic in the summer? A GSHP loop can tap that heat source for hot water heating, even more efficiently than solar water panels on the roof.

13.49

Pool too hot for comfort in the summer? Extract that heat with a WSHP and use that thermal energy for hot water heating.

13.50

Adding thermal storage (a large, well-insulated water tank) to a ground loop system improves system efficiency and electricity costs (with controls to use the storage to lower peak demand for power).