February 1, 2010

Plant Efficiency: Begin with the Right Definitions

Dr. Robert Peltier, PE

The race is on to claim the title of "most efficient coal-fired power plant" on the planet. However, it’s tricky identifying finalists because of the widespread misuse of the term "efficiency" and all those nagging assumptions. Let’s first establish clear definitions and then identify the title contenders.

Most utilities will cover rising demand for electricity by building newer, higher efficiency plants and by adding energy efficiency and conservation (EEC) measures to their systems. EEC options include measures taken on the demand side (such as insulating residences, installing compact fluorescent lights, and advocating for building codes that promote more energy efficient construction practices), demand shifting (industrial process changes and energy storage), and even deploying strategically located distributed generation assets. Another approach is one used by many industrial facilities with a large appetite for electricity and thermal energy: Build a cogeneration plant that simultaneously produces cost-effective electricity, reduces expensive demand charges, and makes the needed thermal energy by recovering waste heat from the prime mover. The most common prime movers are gas-fired combustion turbines or reciprocating engines.

CHP Has a Long History

Cogeneration or combined heat and power (CHP) systems are far from being a new technology (Figure 1). In fact, many of the early electricity plants built at the turn of the prior century adapted their electricity production to systems that coincidentally recovered thermal energy from process heat that would otherwise be wasted. These small cogeneration systems were custom designed for a particular factory, and those designs worked well even without a grid to provide backup power. Fuels, at that time, were principally coal and oil. By the 1930s, the transmission grid began to extend beyond large population centers to provide reliable and relatively inexpensive electricity to the masses. By the end of the World War II, those early cogeneration plants could not economically compete with central station plants, so most were soon shuttered.



1. Waste not. A combined heat and power plant recovers waste heat to produce thermal energy, reducing overall consumption of fuel and reducing air emissions (compared to burning fuel to generate electricity and heat). This technology has been used in industrial and other settings for over a century. Source: EPA


Rising prices for oil and natural gas in the 1970s pushed many U.S. industries to reconsider onsite cogeneration of electricity and thermal energy to reduce their internal energy costs. The Public Utilities Regulatory Policies Act (PURPA), signed into law in 1978, encouraged a return to CHP as a national imperative. PURPA, since repealed as part of the Energy Policy Act of 2005, required utilities to interconnect "qualified" cogeneration plants owned by non-utility companies that met a certain threshold "efficiency." PURPA may have lost favor in the power industry, but it did give birth to today’s independent power and merchant energy businesses.

European utilities learned the value of CHP systems earlier than the U.S. and routinely constructed power plants that double as thermal energy suppliers to local cities and towns. Utility-scale district heating (and cooling) is the norm in Europe but remains an oddity in the U.S., the ConEdison district steam system in Manhattan (which supplies heat, hot water, and air conditioning to 1,800 customers via 105 miles of mains and pipes) being the most notable exception.

Another difference: Most European CHP plants were built by government-owned utilities committed to district heating — much different than the U.S. utility model. A good example of a modern CHP plant is POWER’s 2009 Marmaduke Award winner. Den Haag (The Hague) power plant (profiled in the August 2009 issue) generates thermal energy from two gas turbines for the city’s downtown district heating system. Today, thousands of publicly and privately owned CHP plants in Europe produce reliable and economic electricity and thermal energy for their customers.