Low tech solution helps increase output from peaking plants

Over the past several years, a number of high-tech solutions have emerged as the industry endeavors to make the existing grid stronger, more resilient and more efficient, including synchrophasors, distributed series reactors and devices that enable dynamic line ratings.

But not all solutions are high-tech. At least one solution marries two existing technologies – including one that is decidedly low-tech – to maximize the output of combustion turbine (CT) generators at precisely the time the additional output is needed most.

A company called Turbine Air Systems is producing a solution that uses well-established air conditioner-like technology to reduce the ambient temperature of air before it is fed into a turbine that runs a peaking generator.

“A combustion turbine’s nameplate rating is based on an ambient temperature of 59 degrees Fahrenheit,” a company representative told TransmissionHub at the 125th Annual Meeting of the National Association of Regulatory Utility Commissioners (NARUC) in Orlando, Fla., last week. “However, the times when they’re needed most is when the temperatures are quite a bit higher – like 105 to 110 degrees.”

According to the company’s literature, 19,000 MW of nameplate peak power is available from gas-fired CTs installed across the country, but that figure drops as the temperature rises.

Because hotter air is less dense than cooler air, turbines have to work harder to compress the air they take in, and therefore become less efficient, resulting in a loss of electrical output in excess of 30%, according to company documents. By utilizing the approach the company calls turbine inlet chilling, the turbine is supplied with cooler air, enabling it to function more efficiently.

One such system, installed at an 1,100 MW power station in Pennsylvania, resulted in a significant increase in power output on a 95-degree day compared to the CT’s output without cooling the intake air, according to company documents. Prior to employing the cooling technology, the power station was only producing 87% of its nameplate capacity. Using the cooling system reduced intake air temperature to 50 degrees, actually allowing the turbine to exceed its rated capacity, and lowered the unit’s heat rate as well.

Named Generation Storage, the company’s system couples what is essentially a large radiator placed in front of the turbine’s intake with a reservoir filled with chilled water, then pumps that water through the radiator to reduce the temperature of the air going in to the turbine to increase its efficiency. At night, when temperatures are cooler and power demand as well as prices are lower, the system circulates the water through a chiller. That water is then stored in a multi-million gallon storage tank for use when temperatures increase.

Since the principles are well known and the technologies are familiar, one question that may arguably arise is, “Why wasn’t this solution developed earlier?”

“The people who make the turbines – the GEs (NYSE:GE) and Siemens of this world – are not the same people who make cooling units,” the representative said, noting that companies that manufacture air conditioners don’t have much interaction with turbine manufacturers.

The company presented at NARUC in an effort to educate utility commissioners about the value of thermal energy storage to their grid, but at least one commission already appears to recognize that the system has worth.

On Nov. 20, the company announced that the ERCOT board of directors approved the company’s generation storage as an “energy storage resource” eligible for wholesale load treatment along with other storage technologies, which allows gas-fired power plant operators in ERCOT the ability to use wholesale priced power when the plant is offline, or when the cost to generate is higher than the wholesale price of power, to charge the thermal energy storage tank associated with the energy storage system.

Finally, maximizing existing assets arguably makes fiscal sense.

“The typical solution has been to build more peaking plants,” the representative said, but noted that the cost of building new CTs is generally estimated at $250 to $300 per incremental megawatt. Accordingly, inlet cooling can provide a cost-effective alternative that allows generators to get the most out of existing units and avoid the time lag, capital expenditure, and regulatory and siting hurdles that can accompany the building of new generation.