Combined heat and power (CHP) plants (also called “cogeneration”) provide both onsite electricity and heat (and, in some cases, cooling). As energy prices and concerns about climate change continue to rise, they have been trumpeted as a way to cut energy costs and emissions. Utilizing heat produced during power generation, the overall thermal efficiency of CHP (total Btuh in vs. useful Btuh out) may reach or exceed 75%, much better than the ~40% for central power plants (which dump their waste heat). Since the 1970s, hundreds of industrial facilities with processes that can use such heat have installed CHP systems.
In an attempt to duplicate that success, many small-scale CHP systems (typically under 1 MW) were installed in California in the years following its 2001 power debacle (which saw brownouts and skyrocketing power prices). Over two hundred such systems were located at commercial and institutional facilities, most using either natural gas-fired microturbines (MT) or internal combustion engines (ICE).
Disappointing ResultsA February 2007 study of 47 of those systems, however, showed a relatively low overall average thermal effi-ciency - under 37% - for such units. Depending on the price of power from central station plants and that of natural gas, that could result in a net higher cost of energy instead of a savings, and no net reduction in emis-sions.
While no single cause for this disappointing performance was found, one of the major issues related to the initial qualification of facilities for CHP operation. In many cases, the necessary coincidence of heat and electric loads was missing: too often, high thermal loads (e.g., space and water heating) occurred when electric loads were low or mod-erate, and vice versa. Since the waste heat is created simultaneously with power production, heat not needed must be dumped since it cannot be economically stored, resulting in a much lower overall thermal efficiency.
The California study indicated that some contractors failed to check the simultaneity of existing power and heat loads when determining which facilities could best take advantage of CHP. This problem was worse with MTs than ICEs due to MT’s relatively lower electrical production efficiency (which produces more waste heat/kWh than ICEs).
Other Problems FoundThe study also found:
- Relatively low capacity factors (i.e., actual electricity output divided by theoretical output if running 24/7) due to unplanned shutdowns.
- Overstatement (at the design stage) of likely hours of operation, resulting in erroneous savings calculations. Some CHP vendors push the notion of 24/7 operation, even though off-peak power may cost less to buy than to generate, and natural gas spot market prices (if no firm contract is in place) may make running a CHP plant uneconomical at certain times of the year.
- Generators did not always provide their rated kW capacities, due either to a failure to account for parasitic loads (e.g., cooling towers) or issues with the engines (e.g., high ambient air temperatures lowering power production efficiency).
- Mismatched or inappropriate choices for heat exchangers. In some cases, units were not sized properly or were not well-suited to the working fluids used in heat transfer. When a heat exchanger fails, efficiency drops and/or the entire system shuts down.
- Some systems were oversized relative to the thermal needs of the facility. The study suggested that systems in-stead be designed for the minimum electric and thermal loads to ensure maximum use of power and waste heat from the CHP plants.
Lessons LearnedThe California study is the first to review a large number of small-scale CHP systems. It contains many valuable lessons for those considering such plants for their own facilities. While many of the systems studied were partially funded with state/utility incentives, the results indicate that such support is no assurance of good re-sults. Since firms installing such systems still bear most of the costs, their investment in such equipment should be informed by these field results.
The New York State Energy Research and Development Authority (NYSERDA) has also funded many small CHP systems, but has not yet studied them as a group. When asked to comment on the California results, one official responded that such systems are “still really just experimental at this point.”
A very good synopsis of the study may be found athttp://www.distributednrg.com/de_0711_energy.html. The full study, performed by Itron under contract to the California Energy Commission and several utilities, may be down-loaded for free athttp://www.itron.com/pages/news_articles_individual.asp?nID=itr_014829.xml.ES