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November 07, 2006



This looks like something Primary Energy would come up with.

Unfortunately, neither the press release nor the company web site have many details on how this works.  Thermodynamically, it would be more efficient to pre-heat the natural gas before the expansion turbine; this would yield more turbine work than expansion of cold gas followed by re-heat.


Using expensive and CO2 producing natural gas in boilers to reheat the natural gas after the cooling effect of reducing the pipeline pressure to local distribution pressure is a sorry waste.

Using the turbine to recover energy from the compressed gas is a step in the right direction, but then operating the fuel cell to provide heat to the cold gas is problematic.

A geothermal heat pump that runs directly off that turbine and uses free ground heat to reheat the natural gas is a much more efficient setup.

It is nice to see utility scale fuel cells get some investment and publicity, but this application is flawed.

Just like the wasteful oil industry practice of using oil byproducts to provide process heat for oil refining. That approach is burning up 20% of the oil we use.

And how did that natural gas in the pipeline get up to operating pressure? Natural gas powered internal combustion engines driving huge compressors. Burning more natural gas at very low efficiency.

All process and pipeline transportation energy possible for expensive, CO2 producing fossil fuels ought to come from renewables instead.


I am no expert in thermodynamics, but from what I see, any energy savings in this system come only from being able to make effective use of the waste heat from the fuel cells. Since the turbine extracts energy from the process of lowering the pressure of the natural gas, the temperature of the gas exiting the turbine would be lower than the temperature of gas exiting a simple expansion valve. Thus MORE heat is needed to warm the gas up to distribution temperature. However, with this system there is a use for the rather low quality heat from the fuel cells.

IF one is going to use natural gas for generating electricity, this is probably a good way to do it, since the waste heat gets used to do something that needs to get done anyway (reheating the gas), rather than just getting thrown away.

If, on the other hand, one only wants to lower the gas pressure and reheat it, a geothermal source heat pump run by the turbine would be the most efficient (as Engineer-Poet has pointed out).


Don I think renwable energy powered heat pumps operating from geothermal and waste heat applied to refining, processing, distillation and many other heat intensive industrial processes could save a large portion of energy now provided by combustion.

When the wind and sun are providing excess energy switch on the refining or distillation plant in effect storing that extra renewable energy in the refined or distilled product.

When demand and supply dictate it, shut the plants down for awhile to adjust demand.

Along with geothermal home and building heating, I think this industrial use of heat pumps would save one third of the power we now get from cO2 producing combustion of expensive fuels like fuel oil, natural gas, and propane.

Jim from The Energy Blog

These systems have to run reliably for 24/7 365 days a year so no renewable source will do that without a week or more of expensive energy storage. You could supplement the grid with renewable power at a central location, but I don't think that is what drx is talking about.

They provide the reheating of the gas as well as producing the electricity, most of which they can sell, as the parasitic loads are likely to be small. They still need back up power, presumably from the grid to operated the valves. I don't know how they heat the gas in case of failure of the fuel cell, they say they have eliminated the boiler. Heating from the grid with electricity would seem to be almost expensive as having a boiler.


Jim, the likely market for this system is existing pressure management facilities which will already have the necessary heating rig. The existing heater can back up the fuel cell; when they say "eliminated" I believe they mean functionally eliminated when the fuel cell system is running.


FWIW, most gas pipelines appear to use electric compressors.  This would effectively make them coal-fired in much of the USA.  Low efficiency makes little sense; why would a gas company waste energy, especially if it meant losing product?  But on to the thermodynamics of this scheme (which I am qualified to comment on).

The ratio of specific heats of methane is 1.27; starting at 293 K and expanding by an 8:1 pressure ratio with 80% efficiency, the outlet temperature would be 209 K - about -64° C.  To get gas through the same expander at 293 K outlet temp, you'd need to pre-heat to ~410 K - about 137 C.  The expansion work from the turbine would be about 40% greater in the second case (and so would the heat input required to bring the gas back up to 293 K).

A heat pump wouldn't do so well at such high temperatures.  On the other hand, an SOFC or microturbine's exhaust would be more than warm enough to preheat the gas.


No doubt I'm missing something, but instead of preheating and then expanding, why not expand and then apply heat? While the turbine would be extract less energy this way, there might be enough to still run a heat pump. All I can think of is that there might be some formation of liquids (or solids - think water vapor) on the cold temperature side before reheat.

If using a heat pump for the heat source, a mitigation strategy would be to de-superheat the compressor output at the turbine inlet (warming the turbine input gas), and then heating the turbine outlet gas by condensing the refrigerant of the heat pump system. If the temperature drop in the turbine is still too large before reheat, a heat exchanger could be inserted at the mid point of the turbine.


Expanding before heating allows ice to form inside the expander.  As the Russians will tell you, this is a bad thing.

The reason not to use a heat pump and a re-heat is quite simple:  the electricity from the fuel cell and turbine is quite valuable, and a pre-heat gets more energy out of the same equipment than a re-heat.  Last, using the waste heat from the fuel cell can raise its effective efficiency from 60% to almost 90%; that's mighty hard to beat.

ariel d. catignas

what is the cost of power it can generate USDper kwhr and return of investment

Ariel D. Catignas

Global warming and pollution control is the most neglected topic in the academe and profit oriented business association around the world ,due to greediness for profit, As a concern engineers we need to review our orientation regarding the technology of harnessing energy which is the major source of pollution and global warming, instead of using the technology for our benefits as a life support system, this technology is now destroying our morality, our planet and our life support system. God gave us natural source of energy from sun, moon, sea, lakes river, gravity, wind ,hot and cold weather, which are already at the surface of the planet. Next, we need to learn on how to balance everything in this planet, God created this planet and the whole universe in balance, any unbalance will created disaster and death , but if we maintain the balance it will maintain and reproduce life support system. God put under the surface of the earth all harmful materials ,if it is under the surface earth,it not for the consumption of mankind, if you get something under the earth,you need also to put something equivalent to replace it and if you take out material from under ground into earth surface and atmosphere see to it that you should put something in surface and atmosphere to absorb all those materials and return it under the earth. We need to focus on technology that will not cause the unbalance on earth's life support system by harnessing renewable source of energy and developing nano technology that will produce super energy efficient machines and using biodegradable materials.

Kit P

Ariel, you may want to stop by the Environmental Engineering Department of your local major university if you think AGW and pollution control are neglected topics. All major energy projects in the US require an EIS to consider the environmental impact.

Mac Sakay

Re engineering our technology to keep our environment safe from Pollution should be the urgent priority of every individual and every nation.

We are in the Dark age of Energy that most engineers have no idea on other source of energy , that only the petrolium and radioactive materials were the efficient and ultimate source of energy with out considering the danger to living things on this planet.

Rich Nations were spending trillions of dollars to find another planet instead of giving funds for the researchers on how to harness energy that keep the clouds afloat in the air , energy source of lighting and thanderbolt. To harness earths gravity without using water and keep our forest intact.

If we didnot take a drastic change on our present technology , we will continue to experience a environmental desaster

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