C. Geothermal district hot water

Most of the city of Helsinki, Finland relies on 50 degrees C. hot water, on 80 degrees C hot water or on live steam for district heating in winter. Helsinki is at 60 degrees North latitude. They use solar hot water farms to supply buildings with heat but there's a problem. Their December 21 allotment of sunshine at 60 degrees latitude is terrible - the sun gets 6 degrees above the horizon at best and clouds on the Baltic Sea can get in the way at that elevation. So, Helsinki burns coal and natural gas. Helsinki would benefit from seasonal geothermal solar heat storage. The availability of sunshine at 60 degrees latitude in summer is immense.

Experimenters are examining the drilling of wells perhaps 6 kilometers into the earth's crust to tap the earth's heat down there. The system might work for a while, but drilling any hole that deep is probably far more expensive than drilling down 100 meters and filling a hill with inexpensive seasonal solar heat year after year in perpetuity.

C1. The physics of having enclosing Russian doll thermoclines in the ground for storing seasonal heat

Preheating the system's new water with any available sunlight can never hurt. At the end of any sunny day the ground around the solar heat transfer station is going to be warm. Also, the geothermal station's outermost Russian doll thermocline is always going to be warmer than the normal ground temperature. Run ice-cold new water through any of these areas to get lukewarm water.

The second layer from the outside of the Russian doll thermoclines might be able to further warm water to 50 degrees C. water. The third layer might reach 80 degrees C. After a hard winter we might need the third layer just to boost the water to 50 degrees C. Generating live steam might be a matter of sending 80 degree water into the fourth Russian doll layer. Hot water and live steam can be sent down Helsinki's existing district heating pipes to heat the city.

C2. Distilling steam from brackish water versus using fresh water

In districts where fresh water is scarce, it's possible to generate live steam from brackish water, from agricultural wastewater, from partially treated sewage or from seawater. Details here. We would need an evaporator that accepted brackish water heated slightly above the boiling point. Part of the water wouldn't evaporate, and that waste brine would have to flow out of the bottom of the evaporator and transfer its heat in stages to incoming brackish water.

Dumping the more brackish residual water without polluting waterways is a known issue. Often the water must be completely evaporated, and then blowing toxic dust can be yet another issue.

Steam can be further heated within the geothermal system before sending it down the steam pipes for district heating, so that no steam condenses within the steam pipes.

C3. District heating and water temperature

Low-temperature seasonal geothermal heat has been successfully tried. Higher temperature storage in water-free rock is new. Higher storage temperatures will be far more efficient for geothermal steam generation.

My mirrored tube delivery system elegantly 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. My 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 dispatchable 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.

C3a. Storing solar heat directly from heliostats

To the right is a 50 foot diameter solar heat storage silo. Multiple heliostats above a college campus parking lot can beam sunlight to its central absorber. A fan moves heat within the rock-filled storage unit. The system delivers both winter nighttime heat and hot water on demand.

The storage facility accomplishes the following goals:

1. Install an array of reflectors above, say, a parking lot and put a heat storage rockbed in the south corner of that lot.

2. Use active hot air heat storage. Air is non-toxic. If a slight amount of air leaks out nobody cares that much.

3. Reduce bird kills. Bird kills are a show-stopping side issue to solve. For that matter, strictly limit solar concentration so that setting fire to anything at all becomes quite unlikely.

4. All equipment needs to survive a major hurricane wind gust without overbuilding anything.

5. Be strict in avoiding any toxic mold buildup.

6. Keep lifetime costs down.

This design uses perfectly flat heliostats (large solar tracking mirrors that apply sunlight to one spot all day), not parabolic heliostats. The real beauty of a single flat heliostat mirror surface is far less fire danger if the single heliostat ever malfunctions in any way. Also, single flat mirror surfaces can be built at a relatively low cost. They don’t have to be individually calibrated for their distance to the storage unit.

The complete system uses a primary and then a secondary mirror reflection to send sunlight into its target. We lose raw solar energy by making a second reflection, but we make up for lost solar efficiency with total system effectiveness. I’m told that high-efficiency 12” squares of 1 mm reflecting glass for the secondary reflector above the rockbed will have a 96% efficient reflection rate but costs $4.00 per square foot, while low-efficiency 12” square 3mm glass tiles for the heliostat will have an 86% efficient reflection rate and cost about $0.50 wholesale.

The pictured heat storage rockbed has a very roughly 30-foot diameter part-spherical reflective dish situated 30 to 50 feet above the heliostat array. The center of the curved reflective dish is tipped almost 45 degrees downward. The spherical-style dish never moves - only the individual heliostats track the sun. The optimum dish curve probably won't be perfectly spherical but will be close to spherical, and it definitely won’t be parabolic. There’s a subtle difference between a spherical dish and a parabolic dish. The former concentrates sunlight moderately well and the latter concentrates sunlight perfectly. I have reasons to settle for “moderately well.”

Given an input of near-parallel sunlight from any specific heliostat, the approximate solar exit focus of the spherical mirror is the bottom of an empty cone within the rock bed or perhaps a bit deeper than the bottom of the cone. The dish takes incoming rectangles of heliostat-reflected sunlight from a wide range of directions and reflects all of them downward into a window array over the top center of the heat storage unit. Incoming near-horizontal rectangles of heliostat sunlight, possibly closer to incoming ovals of sunlight if arriving from a great distance, will get reflected upon the inside of the spherical mirror and concentrated almost vertically down into the solar target below.

The westward ten feet of this spherical dish will act as an optimal target for reflections from heliostats sitting perhaps 45 degrees east of due north, and the eastward ten feet of this dish accepts reflections from heliostats perhaps 45 degrees west of due north. The center of the dish accepts reflections arriving from heliostats sitting due north of the spherical target mirror. Heliostats with an azimuth between due north and 45 degrees away from due north will land their reflections at some specific target spot between the center and the edge.

Closer-range heliostat reflections will be traveling rather upward to the spherical reflector and so they need to land rather high on the circular dish. Longer-range heliostat reflections, especially any heliostats sitting on a faraway hillside, will be arriving flatter to the horizon and so these heliostats will aim a little lower on the dish. In all cases, each individual rectangular heliostat’s reflection must always be aimed to land on one specific rectangular area of the circular-style dish, on the specific rectangle that best reflects 100% of the incoming sunlight down into the target below.

If we find that our target array of windows that cover the center of the circular rockbed is grossly overheated with too many solar reflections, perhaps we need to build a higher elevation and then somewhat wider target window section. This will reduce the total excess heat on any window that accidentally gets splashed with a dark substance someday. Also, we care that birds and voles may be attracted to any concentrator by any insects that have gotten incinerated. One of my goals is to spread out the solar input fairly widely, so that a bird flying through our worst solar input outside of the rockbed might receive a maximum of perhaps ten suns (this limit is subject to change) worth of heat at our worst focal area. We want to avoid incinerating birds in mid-flight. We don't need to melt steel or set anything on fire with exceedingly hot solar input, we just need to maintain safe air temperatures and sunlight readings everywhere on the outside of the heat storage unit.

Air gaps reduce the engineering wind loads required for surviving, say, an unplanned 150 mph hurricane or tornado wind gust. The circular dish would best have overlapping horizontal rows of mirrors, with wind overpressure gaps running horizontally across the circular dish, perhaps once every vertical foot of circular dish. Each foot of mirrored material would then have a 1-inch air gap before the next foot of mirrored material starts. If the incoming sunlight is nearly horizontal and the air gaps are horizontal between mirrors, 100% of the incoming heliostat light will be reflected downward toward the target.

Alternatively, it helps if mirror modules can break off of a frame in 150 mph wind gusts. This would save the frame. Later, new mirror modules could be snapped into place on the frame.

Below the dish is a target array of safety glass windows. Hopefully any insects that land on the windows and any rainfall blown in sideways under the spherical reflector will slide down a tilted window array and will drain off in some drainage channel. The window array sits on top of roughly the center of the heat rockbed.

A cloudy day and winter night insulating door should roll over the target window array to conserve heat as needed. The door must always be unrolled before the heliostats start reflecting in new sunlight after each day’s dawn, after the clouds above have cleared enough for gathering solar heat.

This heat rockbed might be 50 feet in diameter and 20 feet tall, depending on site heat storage requirements. The rockbed is circular, with multiple steel bands running around the outside to counteract the outward pressure of the rockbed's gravel on the walls. The outside of the rockbed may slope inward. so that the weight of the walls counteracts some of the outward pressure of the gravel. Roman aqueducts have functioned for thousands of years, so why not build these heat storage rockbeds correctly?

Below the window in the center of the top of the rockbed is a relative cone or pit dug into the gravel. Parabola-concentrated sunlight goes through the window array at a slight concentration, but then this same sunlight gets even more concentrated farther down the cone. As much of the well-aimed and concentrating sunlight as possible gets to travel downward to heat the stones more in the bottom of the heat rockbed’s cone. Almost all of the light pouring downward through the window array is being concentrated by the circular dish, so that most of the light lands down near the bottom of the cone where it will be more easily trapped. I could see some type of quartz or glass funnel shape at the very bottom helping the concentrated sunlight to sink deeper into the rock bed so that little incoming heat ever gets back out. Some heat re-radiating from the center of the bottom of the cone will hit stones on the side of the pit.

Where extremely high temperatures permit, paint the outer layer of the cone of stones with solar black paint that tends to re-radiate solar heat in wavelengths that bounce off of glass, so that the heat doesn't escape. The bottom of the cone may be so absolutely hot that no solar paint or coating will survive down there. Rocks are pretty dependable to temperatures above 1000 degrees Celsius.

I want a laminar flow of air downward through the pit area. This will transfer convective heat from the surface layer of the pit into the very center of the heat storage area where heat will mainly leak outward in all directions, and later this heat can be transferred to areas further away from the very center.

To achieve this circuit of laminar flow through the rock bed, I recommend the use of stones run through a 4 inch screen, known as 2” to 4” stones or sometimes as 3” stones, down the center core of the heat storage rockbed and along the bottom of the heat storage rockbed. I recommend concrete drain half-pipes along the bottom of the rockbed to increase outward airflow from the center. This can put the extremely hot air generated at the center of the cone right against the bare ground underneath the center of the rockbed.

Use 1” to 2” stones, also known as 1 1/2” stones, through most of the rest of the rockbed. We want a completely even if extremely slow airflow upward through the entire layer of 1 1/2” stones all day. Behind the outside walls and above the floor of the rockbed use layers of sand and possibly pea stone and fine gravel. Sand both holds heat and insulates because sand stops all air passage but sand retains some insulating air.

To get better use of all the natural heat-holding mass in the ground beneath this rockbed, we may want to drive iron rods down several feet into the ground at regular intervals in the center area of the bottom of the rockbed, well away from the rockbed’s outer circumference. These will conduct new heat downward on sunny days and upward at night when we want to draw out some of the extra heat stored in the natural dirt and rocks below our ground level.

Leave a tiny 4-inch air space at the top of the rockbed. Alternatively, leave a full crawlspace if workers need to get in there someday. Let the rockbed's heavily insulated roof, the frame for the spherical mirror and the window array all rest securely on the rock bed below. Build the spherical frame’s concrete supports into the gravel rockbed as needed.

In a circle around the rockbed’s big window array, in the 4” high air space within the rockbed, lay an air barrier. Plant this circular air barrier several feet deep into the rocks using heat-tolerant material, not plastic sheeting that melts.. In a gap in the air barrier, install a heat-tolerant blower fan designed for an industrial exhaust chimney. As a rule the fan motor outside of the heat rockbed stays cool and is replaceable. The motor turns a shaft which turns the blower down in the hot area. The blower has been situated within the coolest possible area of the closed air loop within the heat storage rockbed, for long blower life and for easiest replacement when the rockbed is quite hot. This fan will create an extremely slow loop of laminar air flow down the large rocks in the center of the heat rockbed, quickly outward through the large rocks on the bottom, slowly back up through the relative gravel on the sides of the heat rockbed, to the open air gap under the roof and back to the fan. Without this laminar flow excess solar heat would soon accumulate on the surfaces of the central cone's rocks and too much heat would radiate or convect back up and out the window array.

We also want to use the fan occasionally at night. This will move heat from the entire rockbed area to one spot on top of the heat storage rockbed, not far from the center window, a spot where we plan to transfer the rockbed's stored heat to a fluid such as water, steam or propylene glycol for the district heating of buildings in winter and for pre-heating hot water all year. European district heating typically involves 50 degree C. closed hot water loops, 80 degree C. hot water loops or one way steam pipes. Many American college campuses use a central district heating plant.

Given a nonzero possibility of 10 days in a row of cloudiness, a fossil fuel backup heat generating system will still be needed perhaps 1% of the time. A lowest cost heat storage system might trim off the bottom 50% of district heating needs but not 100%. A huge and really nice storage system could approach 100% heating fuel displacement.

Use a small local well with a sump pump to remove all ground water directly underneath the heat storage rockbed, and then don't let any new precipitation flow into the nearby ground. Then the existing ground should itself act as an additional day or even an additional week of tertiary heat storage as long as ground water flow doesn't carry much of the heat away. The immediate purpose of the top 20 feet of gravel is to facilitate fast solar heat grabbing – all night and long term heat migration within the rock bed is a notably lesser concern. I’d love to discover that we could get away with less gravel.

As I often do, I encourage heliostats designed to spill wind gust overpressures in various ways, in order to lower the total engineering costs of each heliostat in the array.

I encourage wireless heliostats with individual battery packs and with small PV panels - car batteries or other cheap rechargeable batteries might work here. Electricians will always cost big money.

Modern heliostats will focus themselves, rotating elevation and azimuth, within milliradians. I see a balance between heliostat aiming precision and better fundamental collector system design which can compensate for poor aim. The future probably goes to a balancing combination of these two ideals, much as the future of heating goes to a combination of using less heat through better building insulation versus supplying more stored solar heat at night to existing buildings.

I expect that 5G wireless is a huge class action lawsuit waiting to happen. Duck! Older 3G chips will do the job and they'll also broadcast at a greater range than 5G frequencies through heavy rainfall.

I just heard of a unique side issue involving low-to-the-ground heliostats and wild turkeys in New England. It seems that male wild turkeys will rush to attack mirror images of themselves. As a consequence heliostat windows shatter and then the wild turkeys cut themselves to death on the glass. I note this strange side issue because as a rule we must take on every side issue. One solution is not to have mirrors so close to the ground.

C4. Supplying multiple levels of heat

Commercial kitchens need 80 degree C water. Residential showers need roughly 50 degree C water. It's possible for heat pumps to extract winter heat from 30 degree C water. Some districts use steam tunnels.

Given an extremely hot geothermal heat storage, to generate 80 degree heat in a local neighborhood, we might want to use a separate supply and return pipe filled with a hot oil. This would boost our supply of 50 degree C hot water up to a needed temperature onsite.

Sometimes the tar-covered roadway helps to heat the heating pipes. A top layer of tarmac comprising 50% tumbled small recycled glass pieces and 50% tar will capture some of the sun's rays deep within the tar, saving more solar heat, reducing the need for plowing and de-icing the street in winter. However, be careful that the road isn't too hot in summer.

Sometimes a white sidewalk plus urban trellises helps to not heat the cooling pipes beneath the sidewalk. Be careful that the sidewalk isn't too icy in winter.

C5. Current district cooling systems

Most district cooling systems pump seawater or lake water from the bottom of a nearby lake. Especially in high latitudes, deeper ocean temperatures are sufficient for air conditioning purposes and may lower energy costs for large commercial freezer operations. However, numbers of towns don't have a nearby ocean. Worse, climate change is already heating up our oceans and lakes. Using them for a municipal summer heat dump adds to their heat load.

C6. Active geothermal extreme coldness storage

The ground underneath a well-covered heliostat field isn't going to get much solar heat. Why not use one of these fields for storing coldness?

Coldness is, or should be, a commodity. In the 1800s New England merchants cut ice from lakes, stored ice in ice houses and shipped ice to tropical ports for its coldness. A modern city has commercial and industrial freezers. Much of any city's food processing industry needs refrigeration or ice.

C7. In-situ coldness storage needs its own area

I recommend that we learn to store large volumes of geothermal coldness in the ground. I picture a generalized cold utility with pipes, using multiday and seasonal coldness storage, similar to district hot water heating.

Multi-day coldness can often be stored early in the morning in hot regions when solar power is available, when the outside air is cold and when nobody wants that much electricity. Coldness can also be stored seasonally.

If hot water pipes can provide district heating needs on winter mornings, seawater pipes can provide district cooling needs on summer afternoons. Seawater freezes at -5 degrees C. Stored coolness shaves the peak off of July afternoon peak electric power demands.

District below-freezing coldness can be supplied using pipes carrying a mixture of water and an antifreeze chemical. Many auxiliary uses for district stored coldness exist such as food processing plants and data centers. Industries such as dry ice production, liquid nitrogen and liquid oxygen manufacturing can trim their total energy expenditures by starting with district stored coolness and/or with stored coldness. It helps to concentrate major coldness destinations in the same industrial park and to have geothermal coldness storage near this destination.

The world consumes 7 million tons of dry ice each year. I understand that we want the world to use less of this carbon dioxide product and less of many other products, but an environmentally more benign way of manufacturing dry ice would also be useful. We might possibly find a new tool for carbon capture.

C8. Storing seasonal coldness

Heat pumps are most energy efficient when they operate close to their target temperatures. I observe that heat pumps are almost completely inefficient at extracting heat from -20 degree C. air in order to heat 19 degree C. indoor air up to 20 degree C air. At this extreme temperature range it's often better to simply run the same electricity through a heating coil. The same principle is true at lower temperatures. If the winter morning outside air is at 0 degrees C and our target coldness temperature is -20 degrees C, that's not a terrible difference in temperature. Cooling anything from 30 degree C (84 degree F) to -20 degrees C is going to be far more energy intensive.

An extremely low temperature Arctic seasonal coldness site will need to store a vast amount of low-grade geothermal coldness during the Arctic winter, using wind power because solar power isn't available in midwinter. In the Arctic spring, solar power can be used to translate the vast amount of low-grade coldness into a smaller, more concentrated amount of extreme coldness at the middle of the geothermal coldness storage site.

For generating seasonal coldness we need a source of renewable power, it can be intermittent, and we need an initial source of coldness. The best combinations of outside coldness and power are likely to be found on February mornings. December mornings are too sunless to generate much photovoltaic power and January mornings may tend to be too close to peak electricity demand, which often occurs on cold nights just before sunrise.

Seasonal Arctic geothermal coldness storage might be ideal for deep coldness jobs such as dry ice manufacturing. Later in the book I discuss a method of extracting relatively pure carbon dioxide from the atmosphere. Given a pool of extreme seasonally stored coldness, a pipe full of carbon dioxide gas can be frozen into dry ice. Large, well insulated quantities of dry ice could then be shipped to warmer climates by rail cars or by ship.

Most geothermal coldness storage sites will have no ground water problems. The ground is well-frozen.

C9. Seasonally manufacturing coldness

The main technology for manufacturing coldness and air conditioning is a heat pump. Your air conditioner and your freezer both contain heat pumps. Your freezer would be more energy efficient, and it would dump less extra heat into your house's air on a July afternoon, if a source of seasonally generated district coldness was running through the freezer's heat exchanger.

I'll mention phase-change chemicals driven by solar heat as an alternative method of manufacturing coldness from concentrated sunlight. Because concentrated sunlight is piped into an area and can be redirected, it's possible to not move a phase-change chemical. In the solar heating phase, apply the sunlight. In the cooling phase, block off the sunlight and bring a cooling fluid next to the phase-change chemical.

As with geothermal heating, it's best to use geothermal coldness sites to cool a coldness transfer fluid in stages. The outermost bowl of a geothermal coldness site will be cool or cold, while inner layers should be progressively colder. A heavy duty Arctic site should be cold and large enough in the center to permanently store great quantities of dry ice. It's often easier to ship the dry ice south in summer when the sea ice is out.

Dry ice, frozen carbon dioxide, can be manufactured through refrigeration of a stream of nearly pure carbon dioxide gas. The process of refrigeration and freezing uses a heat pump. It takes energy.

Using wind power to cool either air or pure CO2 below -80 degrees Celsius in an Arctic or near-Antarctic midwinter factory might be an energy-efficient way to reduce the manufacturing costs of extracting dry ice (CO2) from the atmosphere. Container ships could bring the frozen CO2 toward lower latitudes in bulk, and could ship pressurized pure CO2 north, during the summer.

C10. Coolness, moisture and toxic mold

The freezer industry has for generations known how to remove moisture from freezer air in order to eliminate frost and ice buildup. Also, air conditioners dehumidify indoor air. If we want to store coolness in a basement floor, in a ground-level concrete slab or deep in the ground, then we must learn how to dehumidify the basement's air. We especially must dehumidify any air touching the surface of the cold concrete. Multiple humidity backup systems are possible. Regular humidity inspections might be part of any installation contract.

Many food processing plants need power both for hot water and for freezing. A store or factory sitting on a large enough concrete slab can consider using part of its slab for heat storage and another part, diagonally across the slab, for coldness storage. Both sections of concrete slab would be covered with insulation. In hours when electricity is plentiful, a heat pump can preheat the hot concrete and the ground underneath and pre-cool the cold concrete, often depending on the outside air temperature for optimal energy use. As the store or factory needs to generate heat or coldness, various heat pumps will draw on these dispatchable heat and coolness reservoirs.

A below-freezing section of ground inherently won't lose that much coldness because it's permanently dry. Without flowing water moving heat and without moisture, dry sand and soil are effective insulating mediums. Ground water will freeze into a layer of ice at all edges of the below-freezing section, keeping the center part dry.

Commercial and industrial building owners have the option of drilling through their concrete slabs to the subsoil for better heat and coldness transfer and storage. New buildings can be built with pre-planned floor slab use.

To integrate seasonal geothermal heating and cooling with our electrical grid's need to generate negawatts during periods of peak demand, we need to integrate a centralized weather forecasting function and electrical grid supply/demand to optimize electrical grid summer and winter performance in rare weather contingencies. To be effective we would need to monetize electrical peak load electricity savings. A market-based savings system integrates consumer and business preferences into these heating plans.