Closed-loop geothermal energy recovery from deep high enthalpy systems | Hacker News Reader

Closed-loop geothermal energy recovery from deep high enthalpy systems

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One very interesting thing about closed-loop geothermal is that it isn't actually a baseload source. It has storage-like characteristics. One can draw more heat from it for a limited time, at a lower overall cost than a system that would be designed to draw that same level of heat constantly. This is due to the finite thermal conductivity of rock around the closed loop. After a "surge" that rock recharges by conduction of heat from further away.

Operating at such deep levels and high temperatures presents many challenges that can be avoided with lower overall temperatures and shallow wells.

Eg: My residential well is at least +32F whilst the ambient outside temperature is -40F. I know this through observation of water flowing out of my tap and an outdoor thermometer. This provides a temperature difference of >72F which can be extracted through simple heat converters and then multiplied by compression to achieve temperatures sufficient for steam production. By exploiting the physical properties of steam condensation vast amounts of energy can be produced during this state change.

I understand this report is directed towards large scale energy production with 30 year+ timelines but believe smaller decentralized solutions have many benefits over these large projects.

I also do not understand why "Closed-loop" terminology is implying that this system is something new or distinct as all geothermal systems to my knowledge use closed loop systems to prevent environmental damage.(Other than possibly a water wheel at Yellowstone?)

>I also do not understand why "Closed-loop" terminology is implying that this system is something new or distinct as all geothermal systems to my knowledge use closed loop systems to prevent environmental damage

Large-scale geothermal systems attempted up to now have generally been of the the type where you frack deep underground rock, then dig two wells into the fracked rock field, pump cool water into one and let it heat as it percolates through, then pump it back out the other.

This as you mention carries the potential for environmental damage, but has been seen as necessary to overcome the problem of rock's slow thermal recharge rate.

The linked paper suggests the rock thermal recharge problem can be overcome by digging not one but several horizontal pipes connecting the two deep vertical wells.

I was not aware they used such methods, I assumed they would use a glycol type solution to achieve better heat transfer and enable the transfer of heated media within a closed loop system of piping or well jackets. I find it shocking that they have been employing the tactics You described above, especially under the guise of a renewable alternative energy that is good for the environment.

I also was not aware of that open loop scheme. Seems like a simple and lower cost, albeit inefficient, solution. Thanks for sharing.

Not all geothermal systems are closed loop. I have an open-loop geothermal system at my house on Cape Cod, MA. It uses the same water pump I use for my domestic water, runs it through the heat pump, and discharges back to the aquifer via another well drilled exclusively for this purpose.

Interesting, do you also take advantage of the excess amount of water being pumped through your system with a reverse osmosis system?

I would imagine this would be of great benefit if You are already moving the water to heat and cool your home.

I had the same reaction about the "closed loop" highlight in the title. An open loop system seems very wasteful and hard to maintain.

Your comment on decentralized application caught my attention. I agree with you. While there are clear merits to gaining economies of scale, there are often diseconomies of scale that go overlooked - which would make smaller decentralized applications more attractive.

The trade largely comes down to lowering require capital expenditure ($/resource generated, e.g. $/kW) in order to gain operating flexibility (which can lower overall operating costs - though those savings are normally excluded from the published $/resource value, hence overlooked). Think of the merits of everyone having a car instead of trying to move people to-and-fro using trains. Sure, you pay less $/mile traveled, cumulative carbon emissions/mile are generally lower, and you avoid wear and tear on your car. BUT, you have to pay for taxis or rental car when you arrive at your destination, so you pay an inflated amout to restore flexibility. The cost for 'last mile' transit certainly isn't reflected in your train ticket cost.

As a side study I've been wrestling with this problem in my industry (commercial ship design) for the past decade. One challenge is that it's hard for a corporation investing in the infrastructure to sell these decentralized units to achieve a profit margin competitive with a large-centralized product.

The solution, I believe, is to figure out how to evalutate and factor in the cost of loss of flexibility in the $/resource calculation. That hasn't been an easy nut to crack.

"By exploiting the physical properties of steam condensation vast amounts of energy can be produced during this state change."

Can you expand on this, or provide a reference?

https://en.wikipedia.org/wiki/History_of_the_steam_engine#:~....

Closed-loop technology applies thousands of meters long horizontal wellbores in the target thermal reservoir that are connected by two vertical wells, one inlet well, and one production well (Fig. 2). A low temperature working fluid is injected from the inlet, heated mostly through the horizontal interval in a geothermal reservoir with proper thermal properties, and the heat energy is harvested at the outlet for heating and power generation purposes.

That's reasonable enough. It's an underground heat exchanger. How hard is that to drill? Can you get the horizontal and vertical wells to meet up?

Output temperatures shown on the graph are in the 30C-50C range, so this is for heating buildings, not power generation.

"this closed-loop system can provide a relatively stable energy production with fluids temperature over 100 C on a 30 year lifespan."

That paragraph is a little better, but 100C is not enough for a power plant. No steam, just hot water. Nuclear power plants typically produce 300C steam, and that's thermodyanamicaly inefficient. Naval steam boilers hit the 600C - 800C range.

There's no reason to stick with legacy water-based generation technology that was designed on the assumption of plentiful availability of fossil fuels for heating.

Fluids with lower boiling points and Stirling engines could be used to increase efficiency.

Recall that the Deepwater Horizon incident started when frozen methane at the bottom of the Gulf of Mexico was exposed to oil at 200C, the resulting methane gas generated enough power to send a column of water 300 feet in the air.

Nothing can exceed the efficiency of the Carnot cycle, which is 1-T_cold/T_hot. If T_cold is the room temperature ~ 300 Kelvin, and T_hot = 100C = 373K then this maximum theoretical efficiency is 1- 300/373 ~ 20%.

Horizontal drilling is very expensive.

> A reservoir rock at 200 °C will produce over 120 °C fluids, while a 100 °C reservoir can only produce fluids around 78 °C after 30 years of operation.

Although expensive we have the materials to go much deeper. If we could get to 1400C we could make concrete almost for free.

In Otaniemi they drilled to 6.4 km and got 125C rock. https://agu.confex.com/agu/fm18/meetingapp.cgi/Paper/437325

Current deepest hole is 12 km. Temperature goes up 18 C per km. I think the hole would get quite technical if we tried to go to 70 km deep...

In Iceland that's a bit easier right?

I recall hearing during the Deepwater Horizon incident that the leaking well was a mile deep and the temperature at that depth was over 200C

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