A Quick Feasibility Study on Home Pumped-Hydro Storage + Geothermal

If you follow the world of green energy, you will know there is a basic need to store excess energy from wind and solar in order to provide power when there is no wind or sun. Such systems also offer the potential for microgrids – ie being able to keep small parts of the grid, like just your home, powered even when the main grid it is connected to is down (due to storms, fires, etc). However there is a big split between how commercial systems store energy and how existing home energy systems store energy. Homes mostly use large battery packages, while commercial systems use ‘pumped hydro’ storage.

Pumped hydro storage is really quite simple. You have water turbines running to generate hydroelectricity just like a hydroelectric dam. The difference is that the source is not a fast flowing river, but a reservoir. This reservoir gets filled by pumping water in. So when there is excess energy capacity from solar and wind, water gets pumped up, and when there is excess demand, water is allowed to flow down to turbines.

My overall conclusion, as you will see, is that on our farm we could almost certainly build a grid-tied 1 kW constant power system ‘without too much difficulty’. The concepts are relatively simple but the biggest holdup is the cost-benefit return. Doing all the research, permitting, and of course money spending this would require is greater than the current need we feel for having a home backup energy option.

I haven’t fully explored the idea, but it does seem to me this might become more economically attractive if you are doing a new build of a complete system where you can integrate the solar, hydro storage and a geothermal heat pump. Geothermal systems use ground heat from ground water to provide cooling to a home, and slightly less well, heating. They need a lot less water pump capacity than home hydro, but theoretically I could see such a system being passively integrated to the hydro storage water movements.

A second case where this might be worth investment is a hydro-interested homeowner who has a small creek on their land with some hydro potential already, but not enough for their home powering. Adding solar-powered pumps looping water back to the top could bring the system up to usable sustainable power levels. There aren’t going to be too many homeowners like this, though.

Pros of Hydro Storage

  • Very simple (relatively), extremely well developed and mature technology in the form of water pumps and water turbines
  • Very long lasting, hydro stations life can be expected to be hundreds of years with some maintenance, while a battery pack is a couple of decades at most
  • Safest energy storage type
  • Cheaper at large scale, although probably not cheaper than batteries at the home scale

Cons of Hydro Storage

  • Large land and hill requirements
  • Limited power generation variability (a constant 1kW, for instance, while a home’s needs vary significantly)
  • Not any commercial installation options (although between a well driller, an electrician, and hydro installers (there are a few) you could still get a professional construction of the components)
  • Permits required for well, and probably for a reservoir

A callout to that limited power variability mention above. Most modern homes don’t use too much energy at any one moment, with LEDs, insulation, and energy efficient appliances dropping the load down significantly from what the past. Before getting started on energy storage, it is worth doing an audit of your house and figuring out just what highest energy consumption appliances are, and possibly replacing or reducing them. But some things can never be low energy. Any form of electrical heating uses a very large amount of power. That includes stoves, but also your coffee maker likely using about 1800 watts. A home usually peaks at several thousand watts of energy use, but will can often be just a few hundred watts. Constant even power is exactly what commercial energy providers love about hydro-storage for powering regional grids, but it works differently from how most people actually use their homes.

So for example, if this solution was to be your only power source, you would probably need to size for a minimum of 2 kW constant power, and then ration use of energy intensive appliances carefully. However, most of the time you would be generating excessive power. A 500 watt to 1 kW system is going to be a more practical in size, enough to power efficient refrigerators and lights in an emergency, and most of your baseline energy demand day to day. A practical hydro storage would basically have to be grid tied, firstly to provide peak power demand, and secondly to sell extra power to. Batteries don’t have this limitation, being able to surge to much higher power outputs as needed.

The initial concept

Concept System Walkthrough

The system begins with a solar-powered well pump. They are basically just a DC-powered motor. These systems are widely available and would likely be simple enough to install. Note that the use of a solar-DC pump bypasses the expensive inverter part of a solar installation. Here none of the solar energy goes to the grid directly, only to pumping (having a separate, ‘normal’ solar array could be done as well, elsewhere).

Water is pumped (more on this below) into a large artificial pond dug on a hilltop or hillside (for people with an existing hill stream, this would instead be a uphill, dammed reservoir). Digging this and a massive pond liner are costs here. Note that it is an interesting idea to install the solar panels over the pond in hot climates to reduce water loss from evaporation by shading the surface. In our climate, my concern would be more about it not freezing too much in winter, so digging it deep and narrow, possibly covering with a black tarp, would increase heat retention.

Height, having a steep hill, is absolutely critical in being able to store larger amounts of energy with a small water flow. On our land 20 meters of head seems perfectly achievable, perhaps up to 30 meters would be possible too. A pipe will need to run to the lowest elevation possible, and the turbine is at the bottom here. This turbine becomes the power source, with cables carrying the power then to whatever grid tie system is needed. Turbines produce AC power identical to the grid. My understanding is integration is easier than solar, although the frequency needs to be synced at startup.

Closing the water loop is the last issue. There are a couple approaches. The first is basically two reservoirs, an upper and a lower, with water pumped between them. A water input is needed to top off loss due to evaporation (perhaps this could be just rainwater collection in rainy places, assuming incoming silt is filtered). The second is when using a stream, water flows in and out as usual, slightly regulated by this system. The final approach is properly called an ‘open loop’ system but I will call it a ‘partially-closed’ system. Here the top is a well near the pond and the bottom is a drainfield essentially a simple but large septic system (without the solid waste component, or maybe just for a bit of silt handling). It is an almost closed system in that water returns back to the ground near where it came from but I believe legally is an open loop system because the inlet and outlet aren’t directly connected. Likely most water management authorities could permit it as the net ground water loss should be small. This approach would likely require an overflow discharge for times when the ground is saturated as often happens in early Spring. We have a mostly-dry creek bed that could serve this purpose.

I mention the partially-closed system because it seems to be the most conducive to installing an additional geothermal component. The larger ground loop of the natural water table would warm or cool the water much more effectively. It depends on your climate. Here in Minnesota, water and ground freeze quite deeply, no way the more closed loop systems could stay warm enough. In milder climates, big enough ponds would perhaps be sufficient for heat exchange. Ideally a geothermal system would not require any extra pumps, utilizing the existing water flow, but I am not sure easy that would be to integrate. Likely the home would have to be very close to this system (at top or bottom) for it to work without additional pumps.

Seems fairly simple, right? Well let’s now take a look at how this works with some harder numbers.

Concept System Estimated Calculations

The following system uses estimates and is almost certainly not the most optimized for efficiency or cost, but should be a decent starting point estimate.

My hydro turbine expectations come from the PowerSpout PLT, from a small New Zealand company for small turbines with a helpful online calculator. The estimate here is for 500 watts at 5.6 liters (~1.5 gallons) per second from 20 meters (~66 feet) of elevation change inlet to outlet. I am assuming the efficiencies are accurate – as turbines are just spinning motors, easy to calculate, they’ve put time into including pipe and other efficiencies into the calculator, and Kiwis tend to be more honest than average.

Basically the less elevation you have, the more water flow you must have. If you haven’t processed that already, 1.5 gallons per second is a decent bit of water. That means a 68 mm (~2.5 inch) pipe flowing rapidly. With 86400 seconds in a day, you are talking 127,817 gallons, or just under half a million liters of water required for 24 hours of 500 Watts.

How big a reservoir and how big a solar pumping system is needed depends on how much backup reserve capacity is needed and how consistently sun is available. A massive reservoir would mean the long sunny of days of summer would provide a large buffer for long cloudy spells, but even the ‘smallest’ possible reservoirs here are already pretty massive.

Using very general numbers, about half of the days in a year in MN are decently sunny. The shortest is 10 hours long, the longest 17 hours long. About 2,600 yearly hours of sun is reasonable to expect and perhaps the same again of cloudy weather. Then there are 8760 hours in a normal year.

Taking care to choosing a solar pump that works on cloudy days is important, for example like this review: “The Pro1000V works only with full sun and the 800V works all the time, even on cloudy days.” Basically the sizing of panels to motors means some pumps will work just fine (at a much lower rate) on cloudy days, while others won’t even startup until a high power level is reached. Something like 10-20% of a solar panel’s rated capacity will be present on cloudy days.

This turbine needs 127817 gallons per day for 500 watts continuous, 12 kWh per day.

Assuming a big enough reservoir to store all surplus, the basic algebra works out to 2600 * x [sunny] + 2600 * 0.15 * x [cloudy] + 0 * 3560 [no sun] = 46653097 [total yearly gallons for 500W pump]. Solving for x should be the gallons per hour we need the pump rated at.

X = 15603 gallons per hour in bright sun

That would require 2x of the RPS Pro Series V Lakemaker 5 HP (the biggest) kit which is a combined pump and solar array set. That’s rated for 4.4kW. Two kits at $12,000 = $24,000. These are complete, but not installed, kits.

A circular pond using a 100 foot pond liner, 8 feet deep and 80 feet in diameter would hold just over 300,000 gallons. Enough for 2 days of absolute darkness, or more realistically, perhaps 4 days of cloudy weather.

I have a feeling our 500W continuous power system, 12 kWh per day system would cost around $50,000 using kits and contractors. Well installation, pond construction, piping, cabling, electronics. Price per watt would go down as the system got bigger.

I suspect this cost could be brought down. For example, the 4.4kW solar array could be closer to $5000 for just the panels. $3500 for the DC pump. Turbine $3000. I’ll throw in another $5,000 for pipes, cables, controllers, mounting hardware. $10,000 for a well… Assuming we dig our own pond with our small excavator. A price of $30,000 seems attainable. It might be worth ordering a pump or turbine from Alibab/AliExpress from China – this country is the largest current installers of hydropower right now, so likely are quite decent. There also should be tax credits for this system.

The most expensive part of this system is solar pumping. Perhaps most annoying is the size of the reservoir. There is a fix to both these problems: using a system for only a few hours a day. Running a 2 kW turbine for only two hours day would be enough time for most uses, and could reduce energy costs by providing the most power at the peak prices (usually the evening). A solar array provides power during the daytime, and hydro-solar storage could provide power during the evening. Electronic valves for high pressure are expensive (up to $3000) but a microcontroller is cheap, if you know how to program one. I would personally program a system to open the valve and run the turbine, say, half hour in the morning, hour and a half in the evenings, and also to run whenever water levels are high using level sensors (with a different on, then off again point).

Competitor Analysis

Batteries have two primary advantages: ability to source high current and minimal land requirements. A battery pack can power your coffeemaker easily. And if you disconnected everything else, could produce your morning cup of coffee for many days.

A Tesla Powerwall (13.5 kWh) + a 4.8 kW solar array costs around $30,000 here. Maybe $28,000. Undeniably it is much simpler than the hydrostorage solution: call an installer and pay. There’s warranties. There’s an app. There is much more certainty about your investment.

If you wanted to, hydro-storage could be competitive with battery storage. The 12 kWh system could likely be built for around the same cost ($30,000) as the single powerwall-solar system, and produce similar net power over the course of the year.

The main advantage to the hydro system in longevity. The potential to combine with an existing water source (if present) or a geothermal solution, would increase the attractiveness.

Conclusion and Future Directions

I believe a home hydro storage system is feasible, but will not be a practical choice for most consumers. The two mentioned cases: landowners with an existing stream, or landowners with interest in geothermal power, should at least consider the idea. The wealthy prepper looking for home energy generation, storage, and heating/cooling, might find a geo-hydro-solar combined system to be a neat solution.

There is one major factor here that hasn’t been discussed: research on batteries. Batteries are a massive area of academic, government, and industrial attention right now across the world. Lots of avenues are being explored. Demand is creating economics of scale as well. Already battery systems have become much cheaper and more widely available than just a few years ago.

I think the best decision for our particular place is to wait. An advanced battery system in “10-15 years” combined perhaps with some other renovations seems the most reasonable course of action overall here in 2023.

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