Making the Most of Fuels

In light of recent energy shortages in various areas of the country, end users are searching for new ways to not only save on energy costs, but to recycle waste energy. In this month's M/E Roundtable, engineers discuss the trends and issues end users must understand when considering cogeneration or trigeneration systems.

By Mindi Altman Zissman, Assistant Editor September 1, 2001

In light of recent energy shortages in various areas of the country, end users are searching for new ways to not only save on energy costs, but to recycle waste energy. In this month’s M/E Roundtable, engineers discuss the trends and issues end users must understand when considering cogeneration or trigeneration systems.

CONSULTING SPECIFYING ENGINEER (CSE): What are the main reasons motivating end users to consider cogeneration?

STEINER: They include price, availability of electricity and a desire to be an energy-conscious citizen. People have less confidence in utilities now than they use, to and they want self-reliance.

AHUJA: Ultimately, cogeneration is a way to cut down the energy bill. End users are looking at this technology as they begin to focus on de-regulation and create in-house energy generation and fuel management. In addition, process manufacturing creates heat and steam consumption, which can be used for cogeneration.

MECKLER: Currently in California, end user confusion about both state and federal public policy for generating power supplies at a reasonable cost, and perceived concerns about possible interruptions to its own operations, often go hand in hand. The growth and continuing demand for on-site portable or mobile power generation rentals is clear evidence of such concerns. Cogeneration, however, requires more time for planning, evaluation and construction.

CSE: Are more end users today looking into on-site cogeneration systems than before?

STEINER: End users are increasingly designing cogeneration systems to meet their electric needs. These “inside the fence” projects are emerging because people are concerned with high costs and unacceptable outages, and are responding with load shedding, standby generation or cogeneration.

MECKLER: It’s really driven by several factors, but a primary factor is return on investment (ROI) and perceived concern about outages affecting profitability.

AHUJA: A cogeneration plant is usually within the range of a few tens of megawatts. At this level of energy use, any solution that can reduce or alleviate the energy bill is welcome. There is a move toward a cogeneration solution, but not a rush. The unpredictability of gas prices and concern about its long-term availability often delays the decision, even when cogeneration is economically viable.

CSE: Are the economics of cogeneration improving? AHUJA: The more efficient the plant is, the better the economics. You can get up to 80% overall power plant efficiency in both large and small cogeneration systems. But ultimately, it is the heat profile vs. electricity profile that makes this solution economical. The greater the demand for both, the more economical it becomes. If demand is not simultaneous, higher energy prices make the payback hard to justify.

CSE: What are some of the more complicated technical issues involved in the design of cogeneration systems?

MECKLER: One of the biggest technical issues is selection and matching by size and type of the prime mover—the part of the system that converts waste heat into power, or generates heat and power from energy input. Also, proposed facility location, distance from existing or new load centers, the need for redundancy to assure reliability, staff training and prior cogeneration plant design and operating experience, all add to the technical issues.

MASIELLO: Cogeneration plants are relatively complex in terms of design and operation. There are a number of difficulties that can be encountered. First of all, proper analysis of the piping systems, allowing for pipe expansion and preventing overstressing at the equipment connection points is important. Accurate heat and mass balance calculations that will reflect actual operating scenarios is crucial. Proper synchronization controls and plant operational logic for operating gas turbines, steam turbines, duct burner and bypass duct sequencing is also important. Finally, one must establish the plant operation accurately and early in the project to be sure that the appropriate air permits have been properly obtained as well.

AHUJA: To install a cogeneration system inside an already existing installation is generally tough: footprint is an issue. For brand-new facilities, however, the design is much easier. The automatic control of load management is a technical issue, especially if the power plant is load-following and not base-load operating.

CSE: How do you determine if cogeneration or trigeneration is a good investment for an end user?

AHUJA: The economics are straightforward: One must consider the heating and cooling demand profile—hourly profile and year-long usage—the electricity consumption, demand profile and a comparison of the on-site generation with local electric and gas rates. The closer and constant load profiles are, the stronger the case is for cogeneration or trigeneration.

STEINER: It’s much more than just the economic analysis. Cogeneration requires commitment. An end user must ask himself: Am I capable of achieving appropriate maintenance? Is there space for the system? Is there a reliable use of waste heat?

MECKLER: Trigeneration is a strategy by which building combined heating and power, or BCHP, can result in higher waste heat utilization than cogeneration alone. The result is a faster payout than the comparable cogeneration approach. Incremental costs can range from simply employing a single-stage absorption chiller as a bottoming cycle (for low temperature waste-heat driven cooling) to a more sophisticated integrated hybrid cycle for even greater economies. The decision as to which approach will provide the end user with the best ROI and outcome can only be established by a more critical analysis of probable operating scenarios, which rely heavily upon historical operating information along with end user current and foreseeable BCHP needs.

CSE: How common is trigeneration? How is power generation and energy utilization maximized through these systems?

AHUJA: Trigeneration is not very common. Often based on life-cycle cost and volatility of gas prices, it is cheaper to generate chilled water/ice, or create a cold effect with electricity rather than with gas. However, trigeneration is an option for maximizing the use of on-site generation. Keeping in mind that either hot or cold water can be stored, there are many other ways to maximize energy utilization.

Lastly, if the end user can sell the excess electricity to the grid, the economics take on a different look. In this case, real-time spot market prices and spark gaps in gas and power prices can bring unexpected gains and a short-cycle payback on investment.

MECKLER: Use of trigeneration appears to be growing in some larger buildings with multiple use occupancies or longer daily operating hours—particularly in urban areas where high electrical and somewhat lower gas rates prevail. It is also more common where utilization of available waste heat and minimization of peak electrical demand can result during on-peak periods.

CSE: What are the most common uses of waste heat garnered from cogeneration?

MECKLER: Cogeneration waste heat is used to generate domestic, process and heating hot water or steam for single and two-stage absorption chillers, and for combined cycle power which permits supplemental cooling. This utilizes steam turbines with operating efficiencies that are somewhat dependent upon available temperature pressure levels, etc.

STEINER: Environmental heating, cooling and drying products are the potential users. For example, I have even used the waste heat to heat electrolyte in a copper refinery.

CSE: Is cogeneration becoming more common in smaller commercial applications?

MASIELLO: Cogeneration systems seem to be becoming more popular at college campuses, data centers and apartment complexes. Small facilities that have a simultaneous need for heating and cooling at the same time are likely candidates. There are, however, cost and operational issues for small generators when tying into the local utility grid. Standby charges for power can be very large and single handedly overprice a project.

MECKLER: Where greater availability of lower cost microturbines, or where opportunities for larger combined (i.e. hybrid) operations exist, it’s becoming more common. Such opportunities have created greater end user interest, but at present, it is hardly at the level suggested. Until just recently, such applications were often overlooked.

AHUJA: Currently, the capital cost does not justify cogen for smaller applications. However, small cogeneration could take off with incentives for greenhouse-gas reduction. For small businesses or residential customers, hot water for either space heating or washing is currently the most popular cogen application.

CSE: What are some obstacles inhibiting the popularity of these systems?

MASIELLO: In my opinion, the primary obstacles to expanding cogeneration systems design include increasingly difficult permitting processes and rising costs due to emissions regulations, owners who are not comfortable dealing with generating power and energy and the fact that cogeneration systems may require space in an already existing facility. This real estate may be unavailable or very expensive.

MECKLER: Complications arise with factors such as noise reduction, available gas pressure at the site—particularly for gas turbine drivers—lack of staff experience and the inability to outsource operations.

AHUJA: Maintenance is also a cost issue. Simultaneous heating and electric load availability without major surplus or shortages is a huge issue.

CSE: Have local utilities and jurisdictions become more or less friendly to cogeneration and trigeneration systems in the past year?

MECKLER: Combined gas and electrical utilities tend to be more flexible, particularly when cogeneration facilities are intended to operate in parallel with the serving utility. Often times, excessive safety requirements for parallel cogeneration or end user disappointment with income streams from the sale of excess power, can serve as a barrier.

AHUJA: Municipalities in various regions are different and have different drivers. Large cities and their suburbs, where the electric component is needed most, are less friendly due to environmental concerns. The opposite is true in rural areas. Municipalities are friendlier due to the economic impact: Land usage begets taxes and plant construction means jobs.

Utilities have a different agenda, as they promote the construction of cogen facilities for political and bottom-line reasons. Politically, they are promoting cogen for the public awareness of choice. The bottom line is that they don’t need to add new generation to their service territory, but instead can use the plant as a peaking plant in summer with the ice plant that can be charged at off-peak hours in the summer and can provide cooling on peak hours as well.

CSE: How do you anticipate cogeneration and trigeneration growing in the future?

MECKLER: I anticipate growth, but the rate will depend upon resolution of the issues we have discussed, end user needs and concerns for uninterrupted operations.

MASIELLO: In general, it appears that cogeneration will continue to grow along with increasing power and energy demand. Existing coal-burning plants are aging and will eventually require replacement. These plants, in light of present environmental/emission standards, are not likely to be replaced with coal-fired plants, therefore allowing an inroad for natural gas-fire cogeneration systems. Cogeneration systems are ideal for distributed power arrangements.

AHUJA: Technologies will continue to grow, but at a slower pace for the next several years. Until environmental concerns are alleviated and price stability is seen in the energy sector for both gas and electric, the businesses that can afford a premium in today’s market will be the IPP (Independent Power Provider) and the developers. For most businesses, it is still cost prohibitive, especially the niche market of campuses, hospitals and steel industries. The emergence of low output small commercial and residential prepackaged cogen products, will likely become very popular with increasing concern for preserving the environment.

M/E Roundtable Participants

Anil Ahuja, P.E., general manager, Exelon Service Inc., Westchester, Ill.

Milton Meckler, P.E., principal, P2S Engineering Inc., Encino, Calif.

Sheldon Steiner, P.E., senior vice president, Flack & Kurtz, New York

Joseph Masiello, P.E., C.E.M., senior associate, Joseph R. Loring & Associates, Inc., New York

Mindi Altman Zissman, moderator