INTRODUCTION

The term CCHP (combined cooling, heating, and power) is used to describe all electrical power generation systems that utilize recoverable waste heat for space heating, cooling, and domestic hot water purposes.  The main difference between CCHP systems and the typical methods of electrical generation is the utilization of the waste heat rejected from the prime mover to satisfy the thermal demand of a facility. A schematic of a typical CCHP system is shown in Figure 1. The primary goal of c is to present a more attractive option than traditional power supply methods.

CCHP systems usually are operated by using two basic strategies: following the electric load (FEL) and following the thermal load (FTL).¹  However, in addition to the operation strategies, it is necessary to apply optimization criteria to guarantee the benefits of CCHP systems over conventional technologies.  The CCHP system operation strategy will dictate the loading and fuel consumption of the prime mover and thus the energy consumption profile of the CCHP system.

In the case of the FEL operation strategy, the prime mover is loaded to satisfy the electrical demand of the facility through the power generator unit.  The waste heat from the power generation unit then is recovered to satisfy the thermal load of the facility.  For this operational strategy, if the recovered thermal energy is not enough to handle the thermal load (cooling or heating) of the facility, additional heat has to be provided by the auxiliary boiler of the CCHP system.  For the FTL strategy, the prime mover is loaded so that the recovered waste heat will be adequate to supply the facility with the necessary thermal energy to satisfy the heating and cooling requirements. For this operational strategy the amount of electricity produced may or may not be sufficient to provide the electricity required by the building.  Therefore, if the electricity produced is not enough to handle the electrical load, additional electricity has to be imported from the grid.  In contrast, if the electricity produced is more than is required, the excess can be sold back to the grid.

The choice of a CCHP system strategy largely depends on the specific goal of the CCHP system operation.  Typically, analyses of CCHP systems are based on reduction of operational costs.  However, in addition to cost, CCHP systems can be optimized on the basis of different optimization criteria, such as primary energy savings and minimum environmental impact.²  Therefore, in addition to the operational strategies (FEL and FTL), optimization techniques have to be employed to guarantee the lowest cost of operation, reduction of the primary energy consumption (PEC), and/or reduction of carbon dioxide emissions (CDE).

PERFORMANCE OF CCHP SYSTEMS USING DIFFERENT OPTIMIZATION CRITERIA

A reference building was defined to illustrate the performance of CCHP systems under different operational strategies and optimization criteria.  The reference building, which is in Columbus, MS, is a small office with 465 m2 of floor area and a total consumption of electricity and natural gas of 50,942 kWh/year and 12,380 kWh/year, respectively.  With the building site energy consumption data, a comparison of the CCHP system performance with the reference case as well as the optimization of the CCHP system can be performed.

Figure 2 shows the variation of the PEC, the cost, and CDE with respect to the reference case for the two basic operation strategies.   (In all the figures, a negative number implies reduction and a positive number implies increase.)  Figure 2 demonstrates that the FTL strategy reduces the PEC by 4.8% whereas the FEL slightly increases it.   The FEL and FTL strategies decrease the CDE by 13.8% and 12.2%, respectively, while increasing the cost.  However, FTL increases the cost by only 1.5%, whereas FEL increases it by 12%.  In general, it can be seen that for this city, FTL provides better performance than FEL since it reduces the PEC and CDE with the minimum cost increase.  For the FTL strategy there is an excess of electricity of about 7,000 kWh/year that could be translated into an additional 14% of PEC savings.  The cost savings that could be obtained from the excess electricity have to be estimated for each specific location in terms of whether it is possible to sell it back to the utility companies.

The results presented in Figure 2 can be optimized to guarantee the lowest cost of operation, reduction of primary energy consumption, or reduction of carbon dioxide emissions.  Figure 3 illustrates the variation of the PEC, the cost, and the CDE obtained when the primary energy optimization criterion (PE-O) was applied.  It can be observed that FEL operating under the PE-O reduces PEC, cost, and CDE by 7%, 1.5%, and 13.4%, respectively.  The use of the PE-O for FEL is beneficial since it reduces PEC and cost that otherwise increase when the CCHP system is operated FEL without any optimization criterion.  Similarly, FTL operating under the PE-O reduces PEC, cost, and CDE by 7.5%, 3.7%, and 12%, respectively.  The use of the PE-O for FTL is also favorable since it produces higher reductions of PEC and especially cost, which increase when the CCHP system is operated FTL without any optimization criterion.  Another important issue is that when this optimization criterion is used, all the evaluated parameters (PEC, cost, and CDE) are reduced compared with the reference case for both operation strategies.

Figure 4 shows the variation of the PEC, cost, and CDE obtained when the operational cost optimization criterion (OC-O) was applied.  FEL operating under the OC-O reduces PEC, cost, and CDE by 5.5%, 3.0%, and 8.4%, respectively.  Similar to the PE-O criterion, the use of the OC-O for FEL is beneficial since it reduces PEC and cost.  The use of OC-O for FTL reduces the PEC, cost, and CDE by 7.0%, 4.4%, and 9.9%, respectively.  This optimization criterion is beneficial for the FTL strategy since it provides higher reductions of cost than does the CCHP system operated using the FTL strategy without any optimization.  Using this optimization criterion, all the evaluated parameters are reduced compared with the reference case for both operational strategies.  The CDE reduction obtained when cost is optimized is lower than the reduction obtained when primary energy is optimized.  In general, FTL shows better performance than FEL when cost is optimized.

Figure 5 shows the variation of the PEC, cost, and CDE obtained when the emission reduction optimization criterion (ER-O) was applied.  This figure shows that FEL operating under the ER-O reduces PEC and CDE by 5.7% and 14.8%, respectively, and increases the cost by 2.1%.  In addition, Figure 5 illustrates that FTL operating under the ER-O reduces PEC, cost, and CDE by 6.6%, 1.2%, and 12.9%, respectively.  Similar to the other two optimization criteria, the use of the ER-O is beneficial for FTL since it provides higher reductions of CDE, PEC, and especially cost.  It is important to point out that for FEL, the optimization of the emissions reduction is achieved at the expense of the cost.  This reflects the fact that optimizing one parameter may increase or decrease the other two.

Figures 3, 4, and 5 demonstrate that most of the optimization criteria yield better results than that obtained with a CCHP system operating FEL or FTL without any optimization.  In general, if CCHP systems increase the cost of operation, as long as energy savings and reduction of emissions are guaranteed, the implementation of these systems should be considered.

CONCLUSIONS

The performance of CCHP systems can be improved by using different optimization criteria, such as energy savings, operational cost reduction, and minimum environmental impact.  For the case presented here, it can be concluded that all the optimization criteria yield better results than does CCHP operating FEL or FTL without any optimization. The selection of the optimization criteria depends on the main goal of the CCHP system and has to be analyzed carefully to determine the effect on the other parameters.

REFERENCES

¹ Mago, P. J., Fumo, N., and Chamra, L. M. “Performance Analysis of CCHP and CHP Systems Operating Following the Thermal and Electric Load.” International Journal of Energy Research, 2009, Vol. 33, pp. 852–864.

² Mago, P. J., Chamra, L. M., and Hueffed, A. “A Review on Energy, Economical, and Environmental Benefits of the Use of CHP Systems for Small Commercial Buildings for the North American Climate.” International Journal of Energy Research, 2009, Vol. 33, pp. 1252–1265.