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The physical operations of most buildings today rely on two separate energy systems; one electrical and the other thermal which includes fuel oil, gas or waste heat from an industrial process. The thermal energy is used for heating and/or cooling. Through co-generation, the two systems can be effectively integrated into one, greatly improving efficiency and, therefore, reducing primary fuel consumption.

Conventional electrical generation systems rely on the combustion of a primary fuel to heat water and thereby produce steam. The steam is then used to drive electrical generators. Throughout this process, a tremendous amount of heat energy is wasted.

Although individual companies have only recently begun to install co-generation systems of their own, a similar process has been in use by some major utility companies for many years.

Co-generation enabled New York's Consolidated Edison to enter the steam business decades ago. Con Edison provides many of its customers with steam service by harnessing the steam produced by the generators that create electricity for the New York Metropolitan Area.

While the implementation of a co-generation system initially requires a substantial capital investment, the future savings can be very significant. Two New York City hospitals that recently switched to co-generation attest to this. The installation of the new system at Richmond Memorial Hospital in Staten Island demanded an outlay of $1.7 million but the hospital expects to save $250,000 annually in operating costs. Brooklyn's Methodist Hospital spent $4.9 million to convert to co-generation and expects an annual savings in excess of $1 million.

Further, whereas in most utility generation systems, waste heat is removed from the facility through boiler stacks or steam condensers, a co-generation system uses this waste heat for heating and/or cooling. Figure 1 compares the relative use of energy for each of these systems.

When considering how to use the waste heat, it is important to understand that depending on the method used to generate electrical power, the waste heat is not always in a useful form, i.e., first, it may be at a temperature that is too low for any practical use, and secondly, it may be available only when cooling/heating is not needed.

There are basically three conventional methods to generate electricity.

First, when a steam turbine is used to drive a generator, the turbines can be tapped at a pressure where the steam is still useful. This system is used by utility companies.

Depending on the cycle used even for a condensing turbine, the exhaust temperature is still sufficiently high enough to be effective and directly used for heating although it is not suitable for cooling.

A second method uses a gas turbine to drive a generator.

Waste heat from gas turbine exhaust gases is readily usable through a heat recovery (fired or non-fired) boiler for both heating and cooling. The normal size of a plant using this method is 2MW to 25MW.

In the case of diesel engines, the third method, the waste heat from the engine cooling water is of limited use and the energy from the engine exhaust is difficult to recover.

Diesel generating plants are usually limited to about 4MW.

Figures 2, 3 and 4 show flow diagrams for steam turbine, gas turbine and diesel engines respectively.

Waste thermal energy is most easily used for heating since the temperature of the medium need not be high.

Temperatures as low as 160º F can be used efficiently and even lower under certain circumstances.

Refrigeration, however, cannot utilize temperatures less than about 240º F and for efficient operation, not less than 320º F, equivalent to a pressure of about 100 psig.

Chilled water can be generated several ways. The first, using steam at about 100 psig in a turbine to drive a compressor, second, high pressure absorption using steam at 100 psig or hot water at an equivalent temperature and third, low pressure absorption using steam at about 15 psig or hot water at an equivalent temperature.

The equivalent use of steam for each of the above is 10 to 13 pounds per hour per ton for turbine drive or high pressure steam absorption and 17 to 20 pounds per hour per ton for low pressure absorption.

Co-generation systems can be combined with existing electrical generators and are ideal for facilities, such as hospitals, hotels and large industrial plants that are in continuous 24-hour operation, requiring a constant use for the waste heat.

The efficiency and cost-effectiveness of co-generation systems certainly make them very appealing to companies with large facilities and high operating costs; however, they are not suitable for use everywhere. There are some mechanical systems that lend themselves more to the products of co-generating than others.

Furthermore, since a co-generation system is integrated into the facility's general operations, it must be compatible with the existing systems, i.e. the thermal and power loads should balance and the personnel should be capable of operating and maintaining the system.

Effective co-generation relies on a balance between the amounts of primary fuel consumed and waste heat produced; the ideal ratio is one kilowatt of primary fuel for every two kilowatts of waste heat. When such a balance exists, a co-generation scheme will be able to very effectively and efficiently meet the energy needs of a facility with an overall thermal efficiency of about 80%.

Figure 5 indicates the possible use of energy at various points of a cycle.

Ideally, the co-generation plant should operate at a relatively constant balanced load 24 hours per day throughout the year in order to be most economical. In some cases, utility companies provide for lower rates at night which balance day/night operating demands for the plant.

In addition to the need for a constant output of waste heat, structural and spacial considerations must be taken into account as the large size of such systems requires that they be placed either in the lowest levels of a building or housed in their own separate facilities.

Co-generation is beginning to grow as a viable alternate power source and utility companies are beginning to encourage the use and construction of individual cogeneration facilities. Major utilities had been reluctant to do so in previous years but, lately, as they have been unable to increase their capacity enough to meet projected needs, co-generation has become much more appealing.

The term co-generation is often loosely used to describe methods of reducing demand. In most privately owned facilities which generate their own power, fuel is used to drive equipment directly, i.e. diesel engines and gas turbines which, in turn, drive an electrical generator.

This method is more efficient for smaller plants, requires less maintenance and is easier to operate. Although not strictly co-generation, these systems can also save an owner operating costs.

In this system, a generator can be used to limit peak electrical demand by assigning specific equipment such as one of several chillers in a plant to be run from a generator.

By removing one of the chillers from the utility company, the demand can be substantially reduced during the few peak hours when all chillers are required. This is a particular advantage where utility companies have ratchet clauses where a 30 minute peak could set the electrical demand rate for an entire year. This type of system is simple to operate since there is no electrical connection between the on-site generator and the utility company.

A variation on this system is to operate on-site generators in parallel with the utility company and when possible, to sell the excess power back to the utility company. These systems are far more complicated to operate and design.

AKF expects to see an increased use of co-generation during upcoming years in response to the energy situation in the United States. As the demand for energy increases, we have to find ways to make the most of our natural resources and available funds. Co-generation is an option that uses an untapped resource, waste energy, to help reduce the cost of buying electricity.