F. Active geothermal heat storage with dispatchable electricity generation

F1. Geothermal basics

I favor energy conservation. However, most energy conservation measures are relatively small-scale measures. With due respect for small improvements, I'm sorry, but I only have room in this particular book to discuss major improvements.

F2. Natural geothermal energy

In terms of energy, Iceland is lucky. The steady volcanic action that continually creates the island of Iceland creates surface lava flows that cool in a few years into extremely hot rock formations. The rock formations can stay hot for centuries. An old lava flow might be tapped for 50 years of usable steam power.

Note: Iceland's geothermal wells produce hydrogen sulfide gas pollution.

Deep drilling for hot water is being touted as a theoretical, potential cure. In many places on the planet, drilling a hole 6 kilometers into the ground reaches a source of 50 degree Celsius hot water. However, there may be issues with drilling 6 kilometers into the ground. Does deep drilling get somewhat expensive and chancy? What if the drilled holes aren't stable at extreme pressures? What if only a tiny amount of 50 degree Celsius water can be extracted at each hole? For these reasons I classify deep drilling as one horse in the energy race.

F3. The Drake Landing Housing Development

For the past decade the Drake Landing housing development in Alberta, Canada has been displacing 97% of its own building heat needs. It uses boreholes for seasonal geothermal storage of solar heat. It runs solar heated water in a closed loop down and up an array of 100 meter deep wells. In summer it heats the ground. The deep ground then stays hot enough all winter for the housing development's heating needs, including hot water. The ground itself has an insulating value, an R-value, a resistance to the heat leaking sideways, down or up. This resistance adds up with hundreds of feet of rock or soil. It's also possible to add further insulation at ground level.

Underneath almost every acre of land is a potential geothermal storage site. Why not capture solar heat, store multiple summers worth of heat deep in the ground, then use the heat every winter?

Hills can be particularly useful for storing extreme temperatures, as they can be waterproofed on top. Evaporating water can carry off valuable heat.

F4. Waterproofing the top of a hill

Waterproofing a hill or a plain usually requires adequate surface drainage channels for major thunderstorms and a clay cap. The hill will often be covered with heliostats and sealed from surface precipitation, with drainage channels on the surface for heavy rain runoff. On a plain or below a hill, it's possible to artificially lower the local water table by pumping water out.

For the record, the act of massive seasonal storing and releasing of solar heat isn't going to move the needle on climate change one bit. Just as the moon and the sun look like disks as seen from the earth's surface, as seen from the sun the earth would look like a flat disk 135,000 square kilometers in area. The earth absorbs vast amounts of solar heat in the daytime and then radiates that same heat back up into space at night. If humanity stores even half of the solar power falling on one of those square kilometers, the heating difference will still be rather like a match in the middle of a wildfire.

F5. Active geothermal heat storage

It's possible to focus great amounts of solar light on molten eutectic salt pipes. The heat is then piped deep into the ground. Drilling vertical wells capable of holding two eutectic salt pipes should cost less than digging a custom tunnel. Seasonal storage of heat at 550 degrees C can store heat hot enough to generate steam power in winter.

It would be nice to thaw the salt pipes out every morning by sending our concentrated sunlight down the outsides of these black pipes, within mirrored tubes. We could then displace a solar power tower's use of natural gas for this purpose.

This geothermal heat storage unit below is a series of wells drilled into a hill with insulation on top.

With time, heat will slowly spread outward and downward from the heat source through rock. At the center of a high heat geothermal storage area, eutectic salt pipes carry high heat down in summer and can retrieve moderate heat in winter. The top 30 feet of the salt pipe vertical up-down loops are well-insulated because rock/soil on top of the storage area acts as insulation,

F6. Russian doll thermoclines

The thermoclines under the hill or plain are a series of enclosing Russian doll bowl shapes. Sets of curved wells, each arcing down and up, with one entrance and one exit, are drilled to follow these thermoclines. We know how to drill directionally, but this type of bowl-shaped well drilling hasn't necessarily been tried.

Several of the thermocline bowls wrap around the top of the highest temperature area. The pipes in the first thermocline bowl around the center use a high temperature oil to bring steam-quality heat up. The second thermocline bowl directly heats water to steam in October or close to steam in March. The third thermocline bowl preheats water. The fourth thermocline bowl uses seasonal winter meltwater to keep the surrounding rock at normal ground temperatures. Beyond this, a series of wells transfers normal-temperature groundwater around the geothermal storage area so that neighbors have their normal-temperature well water.

As a geothermal storage site gets used year after year and decade after decade, the peripheries of the site tend to get a bit hotter. This in turn can allow the center of the site to get hotter, to hold and deliver more seasonal heat. Deep heat builds up in particular, as the deep heat has no particular exit if ground water isn't carrying it off.

F7. Groundwater issues

Ground water flow is usually in one particular direction deep underground, and the peripheral heat will tend to migrate in that direction where ground water exists. People living downstream will have warm well water coming out of their tap if we don't pump this water out and replace it with a laminar inward flow of normal-temperature groundwater.

One solution to seeing groundwater carry away the heat is to drain the local groundwater, while pumping groundwater into the ground a reasonable distance away from the geothermal area to maintain neighbors' groundwater levels. Replacing neighbors' well water with city water also works.

When the sun shines, heat pumps can take residual heat out of the groundwater at the very edge of a geothermal storage site and deposit that heat further inward on the storage site.

F7a. Heating a hill, cooling a city

12/14/2024 Here's a method of cooling a hot city, starting perhaps with Phoenix, Arizona, while heating a nearby hill.

The sun is visible in the sky as a yellow circle about 1/3 of 1 degree wide, compared to the 180 degrees of the sky from horizon to vertical and to the opposite horizon. When the circle is reflected by any mirror for a distance of, say, ten kilometers, the reflected light spreads out into a seriously diffused circle shape. The diameter of the reflected solar circle is on the order of 1/100 of the distance from the reflecting mirror, so a mirror above a parking lot 10 kilometers from the target hill would project a seriously diffused circle of sunlight about 100 meters high and 100 meters wide when it hits the hill.

I'm picturing thousands of these heliostats, of solar tracking mirrors, all focused on one hilltop covered with solar heat collectors. Each mirror's sunlight would be miniscule but added all together they could easily equal five or ten suns worth of power, not enough power in total to kill a stray bird, but more than enough over a large collector area to powerfully heat the hill. Thousands of city parking lots and city roofs would lose their sunlight on a hot afternoon. The city would become inherently more livable, with reduced total air conditioning costs. All of that power would be going into the nearby hill for seasonally stored dispatchable power. .

This figure to the left shows how a 40 meter diameter disk of incoming light may be absorbed by a series of four 10 meter high solar collection walls on a hillside. . We don't need to build one tall solar collector if four reasonably sized collectors will perform the same job for us.

We can expect all of the incoming sunlight to arrive at a nearly horizontal angle, so a parabolic collector wall that focuses on a heat collector pipe should work. The collector wall doesn't need to move, as the light always arrives from the same horizontal directions.

F8. Passive geothermal heat storage

Given a wall of perhaps 30x solar heat approaching a geothermal target, the light tube is reflected downward into a tunnel. The nearly parallel light can be compressed somewhat into the tunnel, with some reflection losses due to light polarization. Our efforts to keep the light as parallel as possible pay off here, as we can get an enormous amount of heat down a relatively tiny tunnel. We may want glassy walls here to get the light deep down the tunnel.

Tunnels are liable to convective heat rising back out of the tunnel. To prevent this heat loss, a sort of air heat U-trap is made by curving the end of the tunnel upward. Most of the incoming sunlight should make it around this final glassy curve.

I suspect that if the light tunnel ends in a glassy upward-pointing vee, much of the light will go deep into the vee toward the end, and then little of the radiant heat created would ever get out to the surface of the earth. I'm not an optics expert but as an inventor I'm always on the hunt for reasonable possibilities.

I assume that the heat is stored in a hill covered in a layer of waterproofing, of clay with drainange channels on the top of the hill in case of thunderstorms. The center of a waterproofed hill can be heated to above 100 degrees C, above the boiling point of water. This will allow us to generate steam on demand if we need it. The clay-covered hill might not be that good for growing trees, but my instinct is that the entire hill will be covered with heliostats for capturing and storing solar heat.

Overheating of the bedrock may eventually cause the rock to melt into molten lava. I don't know if reaching this extremely high temperature underground would be a particularly bad thing. The molten lava pool won't go anywhere, it will just stay hot and perhaps grow larger. Some industrial uses need extreme heat. Rock is a good seasonal or multi-year thermal energy battery because there's so much rock and because of the great distances from the center of a hill up to the surface or down to the level of flowing groundwater. Rock is essentially self-insulating.

When light bounces obliquely on the side of a mirrored tube the light gets polarized and half of the light becomes heat. We want to pull this residual heat down the tube with a fan into a low-temperature heat storage area. We need to preheat new water to perhaps 40 degrees Celsius, or when intermittent solar and wind power is readily available we might want to upgrade our 40 degree Celsius heat to 50 degree Celsius heat with a heat pump.

F9. Glass vacuum wall insulation

Vacuum tubes help to conserve heat on solar heat absorbing systems. Glass reflects about 8% of the sunlight per layer of glass so we have avoided sending light through any layers of glass throughout the light collecting system, but at a thermal target we can take a different philosophy.

Inside the solar heat collection chamber, we can push the light through a high temperature quartz glass wall, a row of touching evacuated tubes with an insulating air vacuum between the two layers of quartz glass. A ballpark 84% of the light travels through the quartz and is captured. The wall is on a slant, so that the 16% of the light that gets reflected comes back at an angle, hits a mirror and then heads back through the quartz wall or this heat gets otherwise captured. In the end the system captures nearly 100% of the incoming solar heat and it doesn't lose much heat.

[[draw thick glass so most of the light goes straight through, just a little tiny bit bent for holding a vacuum. it's square vacuum tubes with thin quartz in the middle sections, it's not alternating with a third layer of quartz going back and forth. That third layer would cost too much light.]]

F10. Closing the solar tunnel at night

The mirrored tunnel can be closed off with much insulation and multiple doors to keep radiant heat from exiting back up at night and during cloudy periods.

F11. High temperature geothermal issues

Near-parallel concentrated sunlight can be further concentrated through a relatively tiny hole for minimum net heat losses back up the reflective tunnel. Standard glass mirrors will melt at a relatively low temperature, but we might be able to apply reflective quartz glazes onto the surface of polished rock for making reflective walls capable of withstanding extremely high temperatures. Quartz melts above 1600 degrees Celsius.

If a pool of molten lava is desired, focus light from the hole downward through a small cavern area into the lava pool. The long term storage of extreme temperatures in a pool of molten lava or storage of near-lava temperatures could be useful for high temperature industrial applications such as smelting vats of steel five days a week or melting vats of glass.

A small molten lava pool created deep underground in a bone-dry rock formation should self-seal any possible leakage of the pool of molten lava. If molten lava can ever ooze down into an existing crack, it will reach cooler rock and then harden itself into rock, plugging the crack to further leakage.

Creating molten lava may cause chemical changes in the rock. Iceland reports hydrogen sulfide gas pollution related to the natural cooling lava formations that it taps for electrical power and for district heating. Anyone creating a solar-driven artificial lava pool will have to watch out for such pollution problems.

F12. Results

Our mirrored tube delivery system completely solves the fire ignition and bird kill problems that plague power towers, with some heat losses from polarized light reflections on the sides. Costs per kilowatt-hour will be roughly comparable to solar power tower costs, but with 24/7/365 coverage and without the expense of building a tower. The software needed to focus a heliostat onto a target a mere 20 feet away can be unsophisticated. Our new solar field design could double the heat captured per acre per day versus power tower arrays. The best solar power tower sites have reached a sunny day 3 cents per kwh. We hope to at least match the overall electric rates from power towers, but with 24/7/365 coverage.

A fossil fuel or biofuel heat backup system can supply needed heat during a 10-year or a 100-year adverse weather event, and can also help with peak demand needs. This addition can boost the system toward full 99.9999% supply coverage.

Geothermal heat storage can almost completely solve a multi-trillion dollar field, one that is mission-critical for the climate crisis. Multi-year geothermal heat storage can even cover the extremely cloudy winters that occur on the lee shores of the Great Lakes, and can cover sunless winters above the Arctic Circle. In most cases, local electric companies would never need their wider electric grid connections except during maintenance times. Isolated low-lying coral islands might have geothermal heat drainage issues.

We must design and test the heliostats, the targets and mirrored tunnels, the heat transfer station to eutectic salts and the geothermal site.

F13. Alternative approaches considered

By aiming solar heat directly down mirrored tunnels to geothermal heat storage, we have a short, economical chain of energy transfer processes. Building a photovoltaic field in order to heat a geothermal site is energy-inefficient, it’s more acreage-inefficient, it’s costly in terms of lifetime energy costs and it doesn’t necessarily store the higher temperatures that will be more useful next winter.

F14. Climate engineering job jar

We need research to see if somebody else has found any specific data that we need. We need simulations and we need to sketch 3-d prototypes. We need to build and test various subsystems and scale model prototypes, from small to large, often pausing to solve one problem at a time before spending more money.

We must design and test the heliostats, the targets and mirrored tunnels, the heat transfer station to eutectic salts and the geothermal site.

We want to prepare early for down-hole issues. Unclogging a pipe full of eutectic salts 100 feet below the earth’s surface could be a long, hard job. We have probable solutions.

We want to maximize the expected longevity of our products. Weathering tests get some priority.

Excluding water - deep water ground flow taking heat away, unusual geology, well drilling issues.

Minimizing overheating issues - overheating the sides of mirrored tubes, having mirrored tubes collapse with less damage by having breakaway points and strings to hold the fallen walls sideways to oncoming concentrated light, turning heliostats when an overheating condition is detected

Heliostat network issues - keeping heliostats programmed to properly move to safe positions in case positive network connection is broken

As of this writing, MIT reports preliminary success with an aluminum, sodium and rock salt battery. I'll assume an 80% chance of ultimate success with this battery, that this battery will solve our hour-to-hour electricity storage issues. Given a 20% chance of failure, we should still pursue a number of other horses in the energy storage race.

We still need to solve our seasonal and multi-year energy storage issues. I also note that an aluminum battery will probably be heavier per stored kilowatt-hour than a lithium-ion battery, and that weight is a consideration in automobile-centered transit.

F15. Eutectic salts

I should explain the term “eutectic salts.” “Eutectic” means that the melting point of two mixed solids is sometimes lower than the two melting points of the individual solids. For example, road salt is a solid and ice is a solid, but mixing road salt with ice turns the two solids into a subfreezing-temperature liquid salt water.

“Salts” usually doesn't refer to table salt, sodium chloride. An entire class of chemicals are salts. A 60/40 mixture of sodium nitrate and potassium nitrate has a melting point of 260 degrees C and the molten mixture can handle temperatures up to 550 degrees C. Within this temperature range the salt mixture is a liquid that can be pumped through pipes and into a storage tank. The eutectic salts in many power towers save 3 to 5 hours of high temperature solar heat for early evening electricity generation.

F16. Reimagining the solar power tower

As of 2022 over 3 gigawatts of geothermal steam plants have been built in the U.S. alone. The best solar power towers are economically competitive with the best photovoltaic farms. They can sometimes produce electricity at $30 per megawatt-hour.

Power towers have hundreds of heliostats all accurately reflecting sunlight onto a central black solar target on top of a tower. The target is filled with a high-temperature fluid such as molten eutectic salts, and this hot fluid can be pumped through pipes to a storage vat on the ground or to a steam generator.

This power generation shift more effectively covers the electric grid's peak demand hours toward sunset and just beyond sunset. With current technology, storage of heat is much cheaper and more efficient than storage of electricity.

Power towers have issues. A power tower can set fire to any bird unlucky enough to fly into the zone in front of the solar target where hundreds of solar reflections have been focused, or set fire to a blowing leaf or bag. Solar overconcentration needs to be thought of as an inherent fire hazard. If things can go wrong they will.

Power tower operators are having some success playing prerecorded cries of birds in distress to scare the other birds off. However, farmers scare crows off with the same recordings and sometimes the crows have figured things out, so scary bird recordings might not work forever.

Also, power towers that use molten eutectic salts as a heat transfer fluid don't work whenever the salts have solidified. Many current solar power towers burn natural gas each morning to thaw out the molten salt pipes.

Why not store seasonal solar heat in an existing geothermal storage area, then extract extra dispatchable electricity all winter? By moving from the power tower model to the geothermal model we make the following gains:

The bird kill issue is gone. The issue of setting fire to blowing leaves is gone. We can build geothermal sites in forests and in grasslands.

A power tower is ugly to neighbors, but with mirrored light tubes and geothermal storage the tower will be gone. Power towers cost money and they can have a certain glare to them. Towers require the use of natural gas to thaw molten salt pipes every morning. Hopefully that fossil fuel cost could disappear also because we can tube our solar light straight to geothermal storage.

With piped light, heliostats can be placed on separated pieces of real estate. A light tube can be constructed to cross over a roadway to the geothermal site as needed.

Because heliostats in a field don't have to face a power tower, we can dense-pack our heliostats and so we often can get more solar power out of the same hectare of land.

Unlike a PV farm our heliostats aren't colored black. Neither the field beneath the heliostats nor the air around the heliostats will get hot in the daytime, and so the field's neighbors also won't get hotter in the daytime. We should be a better neighbor for the immediate local climate than a PV farm.

Farms of photovoltaic panels aren't all that efficient in converting sunlight to electricity because they shade each other near sunrise and sunset, causing known PV power generation shading problems. At sunset our heliostat field grabs whatever sun is available. Our heliostats don't have any wires at all, and so we only pay expensive electricians to wire the spot where we install the steam turbines. Our heliostats are replaceable.

A power tower's long-range focusing issues are now disappeared. Our heliostats can use low-cost actuators.

Current power towers save enough heat for after-sunset generation. A geothermal site can store multiple winters worth of dispatchable power and generate all winter and all night, when for-real renewable power prices are high. It would cover Germany's dunkelflaute periods, times with no wind and no solar power. A geothermal site can match winter peak demand and can displace natural gas peaker plants. There might not still be a market for PV farms and for power towers for supplying cheap but intermittent sunny day electricity, if light tubes can deliver sunny day steam turbine power at an extremely low price. PV panels on house roofs will still serve a purpose.

As a result, I see geothermal systems competing aggressively for the total 100% renewable power supply market, and they may squeeze all new power tower construction out of the market.

A solar power tower uses thousands of heliostat mirrors to all reflect sunlight onto a single solar heat-absorbing target on top of a tower. The target usually contains pipes filled eutectic salts. This high-temperature liquid is pumped down to the ground where it generates steam, which drives an electric turbine.

F17. Power tower issues

Solar power towers are competitive in terms of price with photovoltaic farms except for a critical issue. Any flying creature that blunders into hundreds of suns worth of heat can get killed or damaged. When insects get damaged by sunlight, insect-eating birds can be attracted. When the birds get damaged, eagles come looking for food. Power tower operators have been playing tape recordings of distressed birds to drive birds away, but this scare tactic may not work forever as birds tend to adjust to their environments.

Some of a power tower's tracking mirrors can be as much as 200 meters away from the target. Keeping a tracking mirror focused on a target with perhaps 1 meter of play requires a focusing precision within 1/8 of 1 degree. This level of precision can be accomplished but it adds cost to each heliostat's gear system. Bill Gates has been funding artificial intelligence software to keep long-range heliostats focused on a target.

The molten salt mixture that circulates inside the target can freeze. Power tower operators have been using natural gas heat to thaw out the molten salt pipes each morning.

F18. Results

Our solar heat/electricity storage system permanently reduces U.S. and world grid electricity-related emissions to nearly zero. Grid electricity availability and pricing will have an indirect impact on transportation and building heating/cooling emissions.

Late at night, during non-sunny or during peak demand hours, especially in winter, water from the solar pond is heated to steam in boilers with high temperature oil pipes or eutectic salt pipes. This steam runs through turbines to generate electricity.

A fossil fuel or biofuel heat backup system can power the steam turbine during a 10-year or a 100-year adverse weather event. This addition provides full 99.9999% local electric supply coverage.

Our mirrored tube delivery system completely solves the fire ignition and bird kill problems that plague power towers. Costs per kilowatt-hour will be roughly comparable to solar power tower costs, but with 24/7/365 coverage and without the expense of building a tower. The software needed to focus a heliostat onto a target a mere 20 feet away can be unsophisticated. Our new solar field design could double the heat captured per acre per day versus power tower arrays. The best solar power tower sites have reached a sunny day 3 cents per kwh. We hope to at least match the overall electric rates from power towers, but with 24/7/365 coverage.

Our solar heat/electricity storage system permanently reduces U.S. and world grid electricity-related emissions to nearly zero. Grid electricity availability and pricing will have an indirect impact on transportation and building heating/cooling emissions. Our solar heat storage system may reduce industrial heat emissions toward zero.

Without a breakthrough, batteries will no longer be able to compete for the stored electricity market.

[https://en.wikipedia.org/wiki/Cost_of_electricity_by_source]

Batteries often have mining issues. The world’s supply of lithium is limited. Hydrogen fuel cells have various hydrogen storage issues. Hydropumping has several secondary environmental issues, it loses 20%-30% of its energy per cycle and usable mountains with flat tops are uncommon. Compressing air into an abandoned mine requires a nearby abandoned mine.

Natural gas peaker plants are maintained solely for the purpose of handling peak electric demand. Natural gas is mostly methane gas, which is a potent greenhouse gas. Each “dry” fracked natural gas well can leak millions of cubic feet of methane into the earth's atmosphere through a vent other than the wellhead, and dry holes aren't that uncommon. I happen to worry about great numbers of dry wells dumping methane into the sky a bit more than I worry about millions of small natural gas pipe leaks. In either case, I don't think that anything near the true climate cost of natural gas peaker plants has been priced into the Wikipedia cost per megawatt-hour numbers.

I aim for roughly $30 per megawatt-hour of stored electricity delivered on demand, and an even lower lifetime cost for steady sunny day steam power generation where the heat isn't stored at all. A mirrored tube can directly deliver any desired amount of heat needed to manufacture steam at a steam boiler, in order to run a steam turbine. If partial cloudiness temporarily covers the sky, the light tube system can temporarily divert extra concentrated sunlight from the geothermal storage system over to the steam boiler in order to keep steam pressure constant. PV farms can't deliver rock-steady power and even a power tower has no heat if clouds cover the early morning sky, but this system delivers electricity rain or shine.

Beyond unproven battery technologies, my only real power storage competitor is my own mass-pumping ski lift which has its own web page elsewhere. It’s best to run multiple good horses in a high stakes horse race, in case one horse stumbles.