geothermal electrical generation

[ponderer]

I have seen claims of 400% efficiency in geothermal DX HVAC systems.

 

If you are putting in 100 units of energy and getting 400 units out, could this arrangement be used to generate electricity somehow?

 

I know there is geothermal power generation at underground hotspots, but could any old spot be used to generate power using a DX type ground loop?

 

If you have 400% efficiency, there must be a way to turn that into electrical power.

Edited November 5, 2010 by ponderer

[Danijel Gorupec]

Do I understand you question correctly... you want to spend 100 units of electrical energy to get 400 units of heat, and then convert these 400 units of heat into more than 100 units of electrical energy? This, of course, cannot be done.... but you can still generate some electrical energy (<100 units).

 

You can achieve 400% with a heat-pump only if there is relatively small difference in temperature between hot and cold heat reservoir. Larger the temperature difference, smaller the efficiency you can achieve. The oposite is when you try to generate electricity... smaller the temperature difference, smaller the efficiency. Therefore, those two 'efficiencies' cancel out.

 

Here can read a bit more about heat-pump efficiency.

 

 

[CaptainPanic]

Contrary to what Danijel Gorupec writes, this can be done. Geothermal energy is a form of fossil energy... so there is no violation of our thermodynamic laws.

Just like with drilling for oil, you can drill for heat. You spend a certain amount of energy (say 100 units), and then energy starts coming out (say 400 units - probably even more actually).

 

However, I think that it's misleading to talk about "efficiency" in this case. An oil well could then be 100,000% energy efficient.

 

The power stations exist already. They just pump water into the ground, which turns to steam due to the heat in the ground. That steam then spins a turbine, which is connected to a generator to make electricity.

 

Can you please explain what you want to achieve with the ground loop? I do not understand it.

[Danijel Gorupec]

Ponderer,

 

Can you clarify your question... Do you talk about hot underground water or about cold (you mentioned 'old spot') underground water?

 

Talking about "efficiency" is indeed misleading this case, I agree with CaptainPanic.

 

 

[TonyMcC]

Agree talking about efficiency is misleading. You might as well claim a solar panel is infinitely efficient since you (personally) put nothing in yet get power out of it. I don't agree that Geothermal energy is a form of fossil energy. The heat inside of the earth is due to several factors, but not due to the burning of fossil fuels. The heat originates from the original formation of the planet plus some other factors such as radioactive decay.

Edited November 5, 2010 by TonyMcC

[ponderer]

A DX geothermal system uses a copper tubing loop in the ground through which is circulated a refrigerant.

 

The refrigerant undergoes a change of state to -40 degrees.

 

There is typically 100ft of ground loop per ton of heating cooling capacity.

 

For heating, a compressor powered by electricity, compresses the gas and heat is extracted from refrigerant in the process of the refrigerant changing state to a liquid. The liquid circulates through the ground loop and picks up heat from the ground changing back into a gas before it returns to the compressor.

 

My understanding of the efficiency claims would be that the process extracts 400 units of energy from the ground in the form of heat, for every 100 units of energy used to power the compressor. The energy used to power the compressor is electricity.

 

Heat is harvested from the ground and is continuously replenished by conductive migration. The ground is used as a heat well, instead of a water well. The refrigerant acts like a sponge soaking up water, but it soaks up heat. The compressor is like squeezing the sponge to release the water, or in the case of the refrigerant, release the heat. As I understand it, the energy released during the state change is 400% of the energy required to provide the pressure for that state change to occur. No new energy is created. That energy is taken from the ground.

 

Perhaps I am misunderstanding the basis of the efficiency claims, but my understanding of the efficiency claims leads me to wonder if a geothermal system could be constructed that could power itself by using a portion of the heat transferred from the ground to make electricity, while using the rest to provide heating and hot water.

 

It would be necessary to increase the ground loop capacity to use some for electrical generation, while the rest is used for heating and cooling.

 

The problem is that even using a desuperheater, hotwater temperature only reaches say 125 degrees F max. It is not hot enough to make steam to drive a generator. Could another refrigerant be used in a heat exchange, instead of water, to drive a turbine with a state change at say 110 degrees. Run an additional secondary cooling ground loop for the secondary refrigerant.

 

All you are doing is the same thing that is done with geothermal hotspots, but instead of driving generators with steam heated by very hot rocks, you use refrigerant with a much lower boiling point, heated by relatively cool rocks.

 

Bascially, the whole process would be, to use a compressor to extract the heat from DX ground loops. Use a desuperheater to increase the temperature, and then use that heat to cause a state change in a secondary coolant, which drives a turbine. The secondary coolant exits the turbine and is circulated through a secondary ground loop to change state back into a liquid, before being heated into steam to drive the turbine again. You are making electricity. No solar panels. No wind turbines.

 

Heat extracted from the ground is converted into electricity, in an amount which is greater than the electrical energy required to run the process.

 

This all depends on my understanding of the efficiencies. Perhaps I have it all wrong.

 

It's called binary cycle geothermal power.

 

http://reachfwd.wordpress.com/2010/03/30/geothermal-power-an-underrated-alternative-source-of-energy/

 

The difference is that a refrigerant, a compressor, and a desuperheater are used to elevate the heat extracted from the earth, at an energy penalty, with a further efficiency penalty for the electrical generation.

 

Apparently it all depends on efficiencies. Maybe using 14 degree C ground temperature it is not a suitable solution for large scale power generation, but considering the modest needs to power the actual geothermal unit itself, it might be a suitable solution for that, if the efficiencies in heat transfer and power generation efficiencies can be optimized.

 

Such a system could provide free hot water, and climate control, off the grid.

 

I found more info:

 

http://anz.theoildrum.com/node/4802

 

Low temperature geothermal power is also starting to attract significant interest, as lower temperature water resources are common in many countries (for example, waste hot water produced by oil and gas wells - in Texas alone, more than 12 billon barrels are produced, with oil companies usually re-injecting the waste water into the earth) and new technologies are beginning to appear that allow these resources to be developed commercially.

 

UTC Power has developed a low-cost Rankine cycle system that can convert temperatures as low as 195 °F (91 °C) into electricity. The technology is similar to a steam engine, with steam or hot water vaporizes a hydrofluorocarbon refrigerant that drives the turbine (it has been compared to a "refrigerator compressor running backwards").

 

... time passes...

 

So, now I am investigating boiling points and it looks like ammonia mixtures could be used and have been used to provide turbine generation at low temperatures.

 

Temperature differences between surface water and deep water have been proposed as a source of power generation using ammonia to run turbines. This same concept might be employed for ground source electrical generation.

 

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JSEEDO000103000002000092000001&idtype=cvips&gifs=yes&ref=no

 

 

http://peswiki.com/index.php/Directory:Closed_Loop_Ammonia_Turbine

 

You people are being so helpful.

 

Bye

Edited November 7, 2010 by ponderer

[Danijel Gorupec]

Okay, ponderer, I see you think hard about it.... however, I will stick to my first answer.

 

In first part of your mail you are describing a heat-pump. Heat-pumps can really achieve 400% 'efficiency', but the word 'efficiency' is here used in more loose way than ussual. As CaptainPanic said, using the word efficiency in that way, you could easily say that an oil well has 100000% efficiency.

 

 

The 'efficiency' of a heat-pump strongly depends on the temperature difference between the cold reservoir (underground temperature) and the hot reservoir (your home temperature). Larger the temperature difference, smaller the efficiency. This is by the law of physics, not only by the construction issues.

 

In fact, if the temperature difference is infinitesimally small, you can theoretically achieve infinte 'efficiency'.

 

(Imagine this.... if there would be no 'efficiency' dependence on the temperature difference, then we could easily build a heat-pump that can boost the temperature for 1000000K with 400% 'efficiency'. Not possible, unfortunately)

 

 

With a heat pump, and with reasonably small temperature difference between hot and cold reservoir, you can spend 100 energy units of electrical energy (for compressor) to extract 400 energy units of heat. The energy is not created, but is extracted from the cold reservoir. As a result, the cold reservoir becomes a bit colder, and the hot reservoir a bit hotter (the temperature difference increases). Fortunately, the cold reservoir is large enough (the underground) and this works infinitely.

 

....

 

To generate electricity you also need a hot and a cold reservoir. You must transfer the energy from hot to cold, and extract some mechanical work in the process (used to drive a generator). This is actually the oposite of the heat-pump process.... (here you can read how it is done in a thermal power plant).

 

Again the laws of physics don't allow that we extract all of the energy that is transferred from the hot to the cold reservoir. Only a percentage of it is theoretically possible. In fact, the process is more efficient if there is large temperature difference between hot and cold reservoir.

 

In fact, if the temperature difference between hot and cold reservoir is infinitesimally small, you can only achieve zero efficiency - you cannot make any electricity. This is why you don't see large thermal power plants that extracts heat from the Gulf stream - altough there are enormous amount of heat available, the temperature difference is very samll and the power plant will be very inefficient (even with coolants that undergo phase-change at that temperature difference).

 

 

So, if you have underground temperature of 280K, you can probably heat your home to 300K using a 400% 'efficient' heat-pump. But you cannot then use this same temperature difference (300K to 280K) to generate electrcity with more than 25% efficiency. Therefore your 'efficiencies' will cancel out.

 

If, on the other hand, you use an heat-pump to boost underground temperature (280K) to 1000K, you will not be able to achieve 'efficiency' greater than, say, 120%. During the electrycity generation (1000K to 280K) you will be able to achieve maybe up to 80% efficiency, but this again cancels out.

 

...

 

You also talk about phase-change importance. Yes, the phase-change is very important and it boosts efficiency. But never above the theoretical maximum. Physics is a worthy opponent.

 

 

Please ask, if you think that I can explain my point better.

 

(*Sorry, I write in Kelvins because I don't understand Farenheits, and I am not sure if you/others are fluent in Celsius. Cultural differences.

[CaptainPanic]

In heat pumps, the "efficiency" is called Coefficient Of Performance, or simply COP. That term is used because efficiency is very confusing. Common heat pumps can achieve a COP of 3-4. A common fridge has a COP of about 3.5.

 

What ponderer describes is a heat pump which extracts heat from the earth, and then drives a turbine. The first part exists. The second, I am not sure.

I believe that the term "ground source heat pump" will lead you to existing manufacturers for what you describe as "ground loops".

 

That reduces the problem to how you can make electricity from something that is less than 100 degrees Celsius, or how you can make a heat engine that has a temperature difference of more than 80 degrees C. Both are very difficult. You mentioned a low boiling liquid - which is a possibility (please check safety regulations!). But as Danijel Gorupec mentioned, the efficiency of heat engines is limited by the 2nd law of thermodynamics.

 

Example:

If your heat pump creates a temperature difference of 100 degrees, and if it is a very good heat engine, your COP is perhaps 3. Here's a table of heat pumps. Note that only the theoretical heat pumps, that exist only on the desk of a physicist, but not in the real world, achieve a COP higher than 4 for an 85 degree difference...

For such large temperature differences, you probably will not achieve more than 2-3 in reality.

You put in 100 units of electricity, and get out 300 units of heat... in the best case scenario.

Then you run that heat through the Carnot cycle heat engine (a steam engine, for example). That has an efficiency of:

[math]\eta_{th} \le 1 - \frac{T_C}{T_H}[/math]

[math]=1 - \frac{273}{373} = 0.268[/math] or 26.8%.

The result is that you only create 300*0.268 = 80 units of new electricity. You lose 20.

 

In addition, I do not think it is very likely that you can build a heat engine that approaches the Carnot maximum efficiency. You likely will lose more units of electricity.

 

Now, it's not all bad.

You still have 220 other units of heat, that you can use to heat the house. The principle is called Combined heat and power.

However, you must realize that you will trade 100 units of electricity for only 80 units of new electricity and 220 units of heat (in our best case scenario!). The question is: is it worth the investment to generate a little electricity? Or is it perhaps smarter to get a more simple system which only heats the house, and produces no additional electricity.

 

Finally, if you have natural gas heating, the latest boilers of central heating also produce electricity. It's called "Micro combined heat and power", and systems are already on the market... you can just order one, and have it installed.

Edited November 8, 2010 by CaptainPanic

[Danijel Gorupec]

Now, it's not all bad.

You still have 220 other units of heat, that you can use to heat the house. The principle is called Combined heat and power.

 

I beleive that here you made a mistake (in otherwise excellent post). You don't have these 220 units of heat any more. You had to release them (into the cold reservoir) in order to create 80 units of electrical energy. Therefore you lost 20, and that's it.

 

If you wanted to use the 'combined heat and power' you could make, say, 20 untis of electicity (for this you may spend 75 units of heat), and use the rest (225) for heating... In any case, it is better to spend electricity directly instead of first powering heat-pump and then use another heat-engine to generate electricity back.

Edited November 8, 2010 by Danijel Gorupec

[CaptainPanic]

I beleive that here you made a mistake (in otherwise excellent post). You don't have these 220 units of heat any more. You had to release them (into the cold reservoir) in order to create 80 units of electrical energy. Therefore you lost 20, and that's it.

 

If you wanted to use the 'combined heat and power' you could make, say, 20 untis of electicity (for this you may spend 75 units of heat), and use the rest (225) for heating... In any case, it is better to spend electricity directly instead of first powering heat-pump and then use another heat-engine to generate electricity back.

 

Hmm... yes. You are right.

You cannot do combined heat and power if the cold sink is the garden (which is a lot colder than the house). The waste heat which must be rejected is so cold that it cannot even heat the house. Thank you for correcting me.

Edited November 8, 2010 by CaptainPanic

[ponderer]

Hmm... yes. You are right.

You cannot do combined heat and power if the cold sink is the garden (which is a lot colder than the house). The waste heat which must be rejected is so cold that it cannot even heat the house. Thank you for correcting me.

 

 

http://www.yourownpower.com/Power/grc%20paper.pdf

 

ABSTRACT

Organic Rankine Cycle power production from low temperature resources has inherently a low thermal efficiency. Low efficiency requires increased power plant equipment size (turbine, condenser, pump and boiler) that can become cost prohibitive. The use of ORC power plant hardware derived from air-conditioning equipment overcomes this cost problem since air-conditioning hardware has a cost structure almost an order of magnitude smaller than that of traditional power generating equipment.

Using the HVAC derivative concept a low-cost 200 kW ORC power plant has been developed as a derivative of a standard 350 ton air-conditioning equipment. The corresponding PureCycleTM200 product was introduced in 2004. It uses waste heat exhaust gases and air-cooled condenser equipment. This paper describes the extension of this ORC development work towards power production from moderate-temperature geothermal resources.

 

-snip-

 

Due to high cost of the equipment, the penetration of this technology has been limited to specific niche markets such as geothermal. Moreover, most ORC applications have been heavily subsidized. The reason for high equipment cost is that current ORC systems utilize low-volume power equipment hardware. Waste heat power recovery systems are inherently limited in thermal efficiency due to the relatively low temperature of waste heat. Consequently, a waste heat power generating ORC system requires larger capacity components (boiler, condenser, turbine and pump) for equivalent power output than conventional fuel fired power generation equipment. This causes high overall system cost. Efforts to improve the ORC cost structure by focussing on thermal efficiency enhancements have not been successful in bringing the system cost down to a level that would allow a large market penetration. The absence of fuel cost means that the economically correct metric to be used for waste-heat power recovery systems is its cost per unit of power generating capacity ($/kWel). Better efficiency is only beneficial as far as it results in lower equipment/installation cost since the waste heat is free.

 

http://contractingbusiness.com/refrigeration/Honeywell_refrigerant_converts_sunlight_into_electricity_0518

R-245fa is non-flammable, non-ozone-depleting and has low toxicity. The heat

transfer properties of Honeywell's R-245fa, including its low boiling point of

59.5 F (15.3 C), makes it ideal for use in ORC systems that use low-temperature

heat and waste heat to generate electricity.

 

So is this all a scam?

 

http://www.powerverdeenergy.com/

 

http://endlessenergyinc.com/how-it-works/

 

See ThermalGen Pricing Here:

 

ThermalGen 4T-48/24 (2,000 to 3,000 sq ft home) Generates 48KW excess in 24 hours = 2Kw per Hr

 

Total Material Cost = $29,625.00

 

30% Profit Margin = $9875.00

 

Retail cost of unit = $39,500.00

 

ThermalGen 5T-72/24 (3,000 to 4,000 sq ft home) Generates 72 KW in 24 Hours = 3Kw per Hr

 

Total Material Cost = $38,150.00

 

30% Profit Margin = $16,350.00

 

Retail cost of unit = $54,500.00

ThermalGen 6T-96/24 (4,000 to 5,000 sq ft home) Generates 96KW in 24 hours = 4K per hr

 

Total Material Cost = $43,750.00

 

30% Profit Margin = $18,750.00

 

Retail cost of unit = $62,500.00

 

 

It seems to me that a 365/7/24 system generating 3 kw for a full day from a geothermal source, is better than a 10kw PV system, running at varying efficiencies depending on time of year and weather, for part of the day.

 

If it works.

Edited November 12, 2010 by ponderer

[Danijel Gorupec]

I quickly read the article and liked the idea to use cheap HVAC equaipment to generate el. power from moderate temperature geothermal source. The HVAC equipment is mass-produced and is therefore much cheaper than specificaly designed power generation equipment. This can make even moderate temperature geothermal sources useful for power generation despite the very low efficiency that is achieved (Ranking cycle: small temperature difference -> low efficiency).

 

However, HVAC used that way has nothing to do with 400% 'efficiency' that is achieved when it is used for heating. It makes sense to use HVAC systems for power generation because of their low cost, not because high efficiency.

[pioneer]

A heat pump uses electric energy to extract energy that is already there. As an analogy, say we had a reservoir of water that is 100 feet high, but the overflow hole is at 101 feet. There is lot of potential energy there, but we will need to pump the water 1 foot before we can get the energy out of 100ft of pressure. Theoretically we get 100 times at much energy as we put in. We are not creating energy, just releasing it with a catalyst.

 

The heat pump works on a similar principle, using the potential energy in a thermal gradient instead of a gravity gradient. The heat wants to flow from hot to cold, but is stuck. So, we first need to give it a little push to get it over the hump so we can tap into that potential energy gradient.

[ponderer]

A heat pump uses electric energy to extract energy that is already there. As an analogy, say we had a reservoir of water that is 100 feet high, but the overflow hole is at 101 feet. There is lot of potential energy there, but we will need to pump the water 1 foot before we can get the energy out of 100ft of pressure. Theoretically we get 100 times at much energy as we put in. We are not creating energy, just releasing it with a catalyst.

 

The heat pump works on a similar principle, using the potential energy in a thermal gradient instead of a gravity gradient. The heat wants to flow from hot to cold, but is stuck. So, we first need to give it a little push to get it over the hump so we can tap into that potential energy gradient.

 

I am looking for ways to make this happen at my home. I would like to produce 2-3Kw of power 7/24/365 to produce 48 to 72 kWh per day year round. Batteries can be used as a buffer for transient high demands.

 

I don't find anyone saying anything very helpful in advancing this effort.

[CaptainPanic]

A heat pump uses electric energy to extract energy that is already there. As an analogy, say we had a reservoir of water that is 100 feet high, but the overflow hole is at 101 feet. There is lot of potential energy there, but we will need to pump the water 1 foot before we can get the energy out of 100ft of pressure. Theoretically we get 100 times at much energy as we put in. We are not creating energy, just releasing it with a catalyst.

As I tried to point out (by linking to wikipedia articles in previous posts) there is a thermodynamic theoretical maximum.

The heat pump works on a similar principle, using the potential energy in a thermal gradient instead of a gravity gradient. The heat wants to flow from hot to cold, but is stuck. So, we first need to give it a little push to get it over the hump so we can tap into that potential energy gradient.

No, I believe your analogy is flawed, and also too optimistic.

 

Potential energy of a reservoir of water, and potential chemical energy have similarities. But the use of a heat pump has little to do with it.

With a reservoir of water, you indeed may only need to overcome 1 ft of height to benefit 100 ft.

With a heat pump, you should compare it that all your water is at ground level. In order to gain 1 ft of height, you must lose 1 ft somewhere else.

 

I am looking for ways to make this happen at my home. I would like to produce 2-3Kw of power 7/24/365 to produce 48 to 72 kWh per day year round. Batteries can be used as a buffer for transient high demands.

 

I don't find anyone saying anything very helpful in advancing this effort.

What?

Where in this world is all the necessary theory on heat pumps, maximum efficiencies, practical links to producers and even brief example calculations not "helpful"?

 

You're like someone trying to climb Everest barefoot, in swimming trunks... you pass several people who tell you: "It's a bad idea". And you reply: "You're not helping me to advance my effort". It is true that maybe it's possible to reach the top. But you'll lose more than you gain.

 

So maybe you should consider to rethink your goal, when at least two experts in the field tell you that your current goals are not going to be very good.