Although combined heat and power (CHP) is a hot topic in the industry, it’s not a new one. Electrical generation was founded on the concept of capturing “waste” heat to improve overall efficiency. In fact, the first central power plant in the United States, located on Pearl Street in Manhattan, was a CHP facility. That plant (belonging to none other than Thomas Edison himself) generated power for the local electric grid and sent process steam — after its use in steam turbines — to local manufacturers and buildings for heat. This model was the first of its kind, setting the bar for efficiencies still mimicked today.
Throughout the 1900s, power plants grew in size and typically were sited away from the main users. Those long distances reduced the economic feasibility of sending process steam to an end-user. Over time, developments in technology, diversity of steam use, and government regulations have revived the concept and are pushing combined heat and power into the future.
Combined heat and power, also referred to as cogeneration, can simply be described as the process of generating two forms of useful energy from a single fuel source. Typically, a “waste heat” or “heat recovery” boiler is placed in a hot exhaust path to make steam to drive a steam turbine and/or send to process for heating. CHP plants normally are sited relatively close to a steam user (aka “host”). Power and steam are used on-site, and/or power is exported to the grid. As a result, losses and overall efficiency are optimized.
Another generation technique is distributed generation. Similar to CHP in that the generation is sited close to the host, distributed generation is different because steam is not necessarily a factor. This article will focus exclusively on CHP; however, keep in mind that many of the topics also apply to the ever-growing field of distributed generation.
Heat rates — a measure of efficiency
When steam is a necessary commodity, CHP plants are recognized as the most efficient means of producing electricity and steam. Producing them independently is less than 50% efficient. When produced together, as CHP, they easily reach an efficiency of 70%. Figure 1 displays efficiencies of common operating scenarios. The blue vertical line represents 100% efficiency. The typical method of indicating efficiency is the system’s heat rate, which is simply the fuel energy input (Btu/hr) divided by the electrical output (kW). Starting at the top, heat rates of coal plants are greater than 10,000 Btu of fuel per kilowatt generated — or roughly 33% efficient. A combustion turbine operating in simple cycle (no heat recovery boiler) in the second slot is slightly better than that. Combined cycle plants operate somewhere around 7,000 Btu/kWh — or roughly 50% efficient. The true advantage of a CHP plant represented in the bottom column is the electrical heat rate fuel chargeable to power (FCPHR) of 4,000 Btu/kWh — or 80% to 85% efficient.
When considering FCPHR, the steam comes as a somewhat free by-product. In simple form, the energy associated with the steam sent to process is credited against the fuel energy input to the prime mover. This is true because the steam has an energy content value that would otherwise need to be produced from a separate boiler. This reduces the energy put into the system for making electricity and increases the perceived electrical efficiency. FCPHR is a complicated calculation that considers steam, condensate return, and auxiliary power associated with the send-out steam. It may seem to be somewhat of a fabricated value, but make no mistake — CHP is easily the most efficient way to make electricity and steam. In fact, fuel chargeable to electricity heat rates is analyzed for weeks before a project moves forward — and is sometimes the main motivator.
Environmentally friendly
Steam and electricity are both crucial to numerous operations. By generating them with CHP, a significant amount of fuel consumption and resulting emissions are eliminated. Pulp and paper mills, breweries, pharmaceutical manufacturers, automotive manu-facturers, hospitals, and other institutions rely on steam and electricity to operate on a daily basis. Without producing steam in CHP mode, steam operations are forced to generate steam by burning fuel in boilers, which results in an overall efficiency loss when compared to CHP.
Efficiency isn’t the only factor considered when looking at potential projects. In fact, it’s a factor that is looked at, understood, and then swept aside to focus on all the other factors that dictate a project, including age of existing equipment to be replaced or modified, space availability, power purchase agreements, steam purchase agreements, capital and financing requirements, gas and electric market questions, availability of grid interconnections, and so on. Projects of the 1970s and ’80s were built based on overall average demand that included all connected users. Today’s projects are not only evaluated on power grid needs, but also on steam needs and end-user sustainability.
Other influences
As industry recognizes and embraces the benefits of CHP, how does this affect the grid, steam users, and the general public? Everyone wants cleaner reliable power, but few welcome it in their own backyard. This is one reason generation was located away from users in the first place. Additionally, as smart metering continues to penetrate the market and electrical tariffs are adjusted, how does this shape the CHP market? Perhaps there isn’t one answer yet, but smartly devised CHP plants — strategically placed to take advantage of the CHP efficiency gains while reducing the effect the plant has on the environment — are certainly one of them. End-users should start with the realization that localized power is beneficial to the economy, the environment, and their bottom dollar. To drive this realization, generation studies are performed every day to analyze industry and develop smart, localized power plants.
Many steam and electric users across the country and globe could benefit from CHP, but it does not mean that this approach is always a good fit. Technically and contractually, power plants of this nature have unique challenges — chief among them are electrical and steam demands by the user and the availability of a viable grid tie-in point. CHP plants are sited close to the steam user due to the cost of transporting steam in piping. The electrical tie-in becomes a secondary but critical path step. In the early stages of development, many electrical design points are addressed, ranging from size and location of nearest substation, routing to said substation, redundancy required, and direction of electron flow (i.e., sale/purchase needed from the grid for black start and supplemental power needs).
Grid connections
The terms generator interconnection and interconnection are used by most Regional Transmission Organizations (RTOs) to describe the policies and procedures for interconnecting new generation into the electrical grid under their jurisdiction. There are seven RTOs within the United States. Although the basic requirements for adding new generation are similar, each RTO has its own specific policies and procedures to achieve compliance. Basic goals include maintaining electrical system reliability standards and assignment of costs related to grid upgrade due to generation addition. Meeting the obligations set forth by the RTO to build the generating facilities is just the first challenge. The second is to offer electricity at a competitive yet profitable rate to make the business successful.
Depending on the size of the plant and/or the amount of electricity displaced to the grid, various factors regarding the interconnection are considered. Typically, analysis that looks at viable grid interconnections and availability of the transmission system to carry the new generation is performed. Certain areas of the grid are considered “weaker” than others — that’s when the interconnection study can get interesting. Every situation is different in the sense that the need for reliability begets cost and typically additional regulatory factors.
Take a major utility, for example. It is likely that no expense is spared to ensure the best possible connection to the grid is achieved. This may mean connecting to a local substation by developing rights-of-way, crossing railroads, and jumping through every imaginable regulatory hoop. On the other hand, a smaller CHP plant developed primarily to serve a single user may decide only a single tap is needed. This smaller CHP plant provides the end-users’ energy needs, and a relatively simple electrical connection is used for redundancy only.
Another complication comes in the form of equipment available to provide electricity for a specific user. Combustion turbines are great at ramping up and down and following the load of a user, but that is not always the goal. Sometimes, a generating arrangement is selected that can provide far more electricity than needed, and electricity is exported/sold in addition to feeding the local load. This makes for a great revenue stream but is not always available. Other times, an undersized plant makes more sense. Take, for example, a process that is base loaded but has significant electrical peaks above the base load. One way to address this is to size equipment to manage the base load and purchase from the grid the occasional additional electrical requirements. Understanding electrical tariffs, value of exporting, and overall knowledge of the manufacturer’s processes is key to developing the best-suited equipment for each unique operating scenario.
Steam is a commodity too
There are just as many factors to consider with steam production for a CHP plant. Typically, process steam usage fluctuates over the course of a day, week, month, and year. This is true for almost all operations. For example, ambient conditions dictate district heating needs, moisture content of sugar beets affect steam needs, and demand affects breweries and beer production. Understanding steam usage on an hourly basis is crucial to sizing and selecting equipment. A shortage of steam to run an operation is no different from flicking on a light switch and the room staying dark.
One common CHP plant arrange-ment consists of a combustion turbine, heat recovery steam boiler, steam turbine, and condenser. Ambient air is sent through the compressor section of the gas turbine, mixed with fuel, and combusted to drive the electrical generator. Combustion turbines range from producing a few megawatts up to a few hundred megawatts and can be used for numerous applications.
Furthermore, gas turbines are not the only prime movers to use. Reciprocating engines, micro-turbines, and fuel cells are common technologies used to cover the complete range of CHP users. The combustion gases are then routed through a heat recovery steam boiler and out a stack. This way, energy that would normally go up the stack in simple cycle is used to generate steam, which is then sent either directly to process or first through a steam turbine to make more electricity, and then extracted at a lower pressure to feed the process.
Steam vs. electricity
Depending on the operation, steam and electricity may be inversely proportional to each other. Take, for example, a university campus in the Northeast. As the steam needs (Fig. 2 – red line) diminish when the weather warms up, the electrical requirement (blue line) increases. August is the peak month in this example for electrical needs, as air conditioners are operating. Selecting equipment from a monthly average will not provide the somewhat instantaneous data needed to ensure the lights stay on and the dorms stay warm. This graph only gives perspective on how the generating equipment will need to run at partial load during certain times. As stated earlier — and if the market can support exporting during certain times of the year — this may be a good chance to sell to the grid and operate base loaded year round.
Another operating scenario is a sugar beet processing plant. Sugar beet processing is a great fit for CHP. As the beets are harvested in June, the plant ramps up from minimal summer load for the winter-long sugar processing endeavor (Fig. 3). The steam demand and electrical demand follow each other, making it easier to match electric and steam equipment. This CHP plant is base loaded all winter and into the spring, until the last beet is processed. The monthly data indicates a maximum load of 11MW and 250,000 lb/hr of steam. The equipment would likely be sized for something more than that. Electrically, the 11MW monthly averages do not display the instantaneous peaks and, therefore, the electrical generating equipment would be sized for something more than 11MW. Similarly, the instantaneous steam demand is needed to size the steam generating equipment. Economically, it is always better to run base loaded and at full load, like this sugar beet processing CHP plant does.
What’s left?
CHP is the most efficient method for generating electricity and steam. Conceptualizing, planning, designing, building, operating, and owning are all so diversified that volumes of books are needed to cover it all. Across the industries, some own and operate while others own but don’t operate. Still other owners do neither, and instead have a third-party investor build/own/operate and sell CHP-derived electricity and steam to the user. Key factors to consider are electricity and steam needs, interconnection, steam user location, and a detailed operating cost analysis.
McCormick is an engineer with Commonwealth Associates, Inc., Jackson, Mich. He can be reached at [email protected].
