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Cheng Cycle vs Combined Cycle Gas Turbine

Discussions of conventional and alternative energy production technologies.

Cheng Cycle vs Combined Cycle Gas Turbine

Unread postby Tanada » Thu 29 Apr 2021, 16:40:45

From time to time over the last 17 years this board has existed we have discussed how Natural Gas has been becoming the dominant electric energy supply of choice. In some small way climate change has been a part of this because all of the utility companies have adopted the idea that burning Natural Gas rather than Coal gets them ecological bonus points with those of us concerned about climate change.

Unfortunately the situation is a lot more talk than action because of how they use the natural gas they burn.

Scenario 1) The big utility company pays to have a large diameter gas line built to supply their already existing coal burning power plant. Then they simply switch to burning Natural Gas in the same boiler and steam turbine Rankine cycle set of equipment they already have which is probably old enough to have already recoup its initial investment. This is about 33% efficient in converting thermal Watts of energy into Electrical Watts of energy.

Scenario 2) Very commonly an Electric Utility scraps an small older inefficient Coal burning plant or builds a brand new Natural Gas peaking plant. These plants are very quick and easy to build, the big Gas Turbine manufacturers build these things in a factory one after another in identical sets. You place and order and within a few months the necessary equipment arrives at your designated plant site. These are "Open cycle Brayton" units. They burn Natural Gas or Jet Fuel or a variety of other liquids if properly adjusted by compressing air to high heat into the combustion chamber, then injecting the fuel and burning it. The burning fuel makes the already moving air very hot which causes it to become even higher in pressure under Boyle's Law of physics. This hot high pressure exhaust is then fed through a power turbine that lets the gasses expand cooling and giving up energy as they do so by spinning the power discs. The power turbine is normally on the same shaft as the compression turbine so a small part of its energy goes into compressing more air into the combustion chamber continuously while it also spins a very large generator outside the gas turbine to make electricity. In these open cycle units the exhaust gas is vented up a chimney and the resulting Brayton cycle being higher in temperature than the Steam Cycle is more efficient converting between 37% and 44% (average numbers) of the thermal Watts produced into Electrical Watts.

Scenario 3) A brand new baseload Natural Gas power station is built either to fully replace an existing Coal station or because demand is high enough to justify construction. Because it is a base load station the Utility usually adds complexity to make more money by being more efficient. Typically for each pair of Gas Turbine units installed the very hot exhaust is routed through a common boiler called an HSRG, Heat Recovery Steam Generator. The steam created in the HSRG is then routed through a convention Rankine Steam Turbine system which spins a third Generator creating additional electricity from the waste heat that the simple cycle GT in Option 2 vents straight up the smoke stack. By capturing a portion of the "wasted" heat this Combined Cycle Gas Turbine or CCGT can bump the overall plant efficiency from 44% max all the way to 52% max by combining the Brayton gas turbine cycles with the Rankine steam turbine cycle. Up until the turn of the millennium this was considered the gold standard most efficient heat to electricity power design available.

Scenario 4) The Cheng Cycle power station. Way back in 1974 Professor Dah Yu Cheng of the University of Santa Clara asked himself if their might be a better way of combining the Brayton and Rankine cycles for more efficiency with less complexity. He received a US Patent for his concept in 1976 and in 1982 the first unit to use his Cheng Cycle was installed by the University of Santa Clara. Over the following 20 years he and his company IPT (International Power Technology) continued to modify and refine the equipment using his new Cheng Cycle with over 100 units installed world wide by 2002.

This is how the Cheng Cycle works. Dr. Cheng reasoned that a Gas Turbine already has a high pressure turbine to convert the energy of the moving super heated exhaust into rotational energy. By 1974 when he started developing his cycle new Gas Turbine engines were already running at temperature significantly higher than Steam Turbines ever reach. So Dr. Cheng arranged a manifold of steam vents around the flame holders in the combustion stage and used them to inject a modest amount of steam in a balanced flow all the way around the circumference of the combustion chamber. This had a couple of effects. First off it increased the mass passing into and through the high pressure turbine section of the gas turbine unit which delivered more energy to the shaft driving the generator equipment. Second because high temperature steam is cooler than the exhaust gasses of the combustion chamber and steam requires a great deal of energy to increase its temperature the net temperature of the steam and exhaust fluid flow through the turbine was lower without reducing its total energy. This lowering of the temperature is very beneficial for the component lifetime even though modern high pressure turbine blades are made of hollow ceramic instead of the steel alloys used in WW II.

With the first model modified gas turbine unit in 1982 the Cheng Cycle in its most basic form the power output of the first small Allison 501 unit increased by 70% and the net efficiency at converting heat into electricity went up from 28% in its designed simple cycle form to 39% in its Cheng Duel Fluid Gas Turbine form. Over the 20 years from 1982 to 2002 a long series of refinements and modification took place and the purposely adapted from scratch to get maximum benefit from the Cheng Cycle GE 7H Base load Gas Turbine system has a maximum efficiency rated at 60%. This is a little less than the claimed 64% efficient CCGT, however by eliminating the separate Rankine cycle generator and steam turbine set along with the condenser and ancillary equipment the Cheng Cycle system has a lower complexity and much lower maintenance costs.

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Rather than license the Cheng patent several large manufacturers have developed competing steam injection systems for their gas turbines usually called STIG systems or STeam Injected Gas Turbine. Though not fully as efficient as the Cheng cycle units they do get a significant power boost compared to a straight run gas turbine and some of the systems are very complex with medium pressure steam bled from the Steam Turbine unit used for the injection into the Gas Turbine. These STIG units capture the Rankine steam generator cycle while also boosting the Gas Turbine efficiency and this has GE claiming a 64% efficiency for their newest CCGT equipment sets. Of course all this added complexity means they require a much greater degree of maintenance to operate at peak efficiency. It always comes back to, how much is that last few percentage points of efficiency worth? Going from 33% Rankine cycle to 44% simple Brayton cycle is pretty obviously worth the switch. Making a system Combined Cycle with both Brayton and Rankine turbines to get up to 50% efficiency is also a pretty sound investment, but it is a significant cost increase. Going from 50% to 64% (GE's latest claim) by adding STIG to a Combined Cycle plant is adding significantly more infrastructure but probably still worth it. However when the Cheng system using the same basic Gas Turbine and HRSG system without the Rankin cycle equipment can achieve 60% with less complexity and maintenance you have to wonder at GE's motivation for insisting on their super complex solution for a relatively small efficiency improvement.
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Re: Cheng Cycle vs Combined Cycle Gas Turbine

Unread postby Simon_R » Tue 04 May 2021, 10:26:44

Hi Tanada

I am no expert, but this could have something to do with Ramp curves, a CCGT has a two phase curve and can get productive pretty quickly, so can act as a quasi 'Peaker' or a more baseload station.
With a higher SNSP utilities are more interested in the CRM market and peaker stations.
However even if Cheng had a less attractive curve, then a grid scale battery could always be used for Ramping support.
Do you have a Cheng Ramp curve, for us to look at ?

thanks

Simon
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Re: Cheng Cycle vs Combined Cycle Gas Turbine

Unread postby Tanada » Tue 04 May 2021, 11:05:13

Simon_R wrote:Hi Tanada

I am no expert, but this could have something to do with Ramp curves, a CCGT has a two phase curve and can get productive pretty quickly, so can act as a quasi 'Peaker' or a more baseload station.
With a higher SNSP utilities are more interested in the CRM market and peaker stations.
However even if Cheng had a less attractive curve, then a grid scale battery could always be used for Ramping support.
Do you have a Cheng Ramp curve, for us to look at ?

thanks

Simon


Not sure what you are looking for but this popped up when I did a graph search. The Allison-500KH was one of the systems Dr. Cheng modified for the University of Santa Clara cogeneration plant producing electricity and venting the waste heat for steam heating in the colder months of the year.

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I should be able to change a diaper, plan an invasion, butcher a hog, design a building, write, balance accounts, build a wall, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, pitch manure, program a computer, cook, fight efficiently, die gallantly. Specialization is for insects.
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Re: Cheng Cycle vs Combined Cycle Gas Turbine

Unread postby Simon_R » Thu 06 May 2021, 15:01:22

Hi Tanada

This is a Heat Curve. All thermal stations vary their output depending on Temperature and Pressure.
Normally this is output in Mw against Ambient Temperature/Pressure.

Each Individual generator has a formula which is applied against it, to produce the curve.

Essentially the colder and denser the air, the more oxygen, the more power.

A ramp rate describes how a Thermal Station cycles from 0% to 100% power, they are not linear (esp. CCGT) and can take hours to get there

I will try to dig out an example

thanks

Simon
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