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Realization of the full benefits of implementing the combined heat and power (CHP) concept in buildings hinges upon optimum CHP system integration, sizing, and operation in parallel with, or independent of, the electric utility grid. This realization necessitates assessment of the appropriate CHP design/operation possibilities and selection of the best candidate for a given application. Electrical- and thermal-load-following CHP models are certainly among such candidates.

This paper is essentially an extension of a previous study on a grid-independent, electrical-load-following CHP system for a hypothetical office building. The objectives of this study are to evaluate the thermodynamic performance of a thermal-load-following CHP system for the same building and to compare the results with those of the previous study. Included in the scope of the current work are (1) a parametric analysis addressing the influence of the subsystem efficiencies on the total primary energy consumption, (2) an evaluation of first-law efficiencies at two levels: CHP system and overall system, (3) an estimation of net monthly electricity import/export, and (4) an assessment of how electric utility efficiency affects the overall system energy consumption.

The parametric analysis demonstrated the positive and significant responsiveness of the total primary energy consumption to improvements in the efficiencies of the on-site power generation and building electrical systems for the thermal-load-following model. A similar finding was also echoed by the previous work on the electrical-load-following CHP. The net monthly export of electricity (for the thermal-following-model) occurred during the peak cooling months, when the building thermal loads are the highest. While an increase in the efficiencies of the on-site power generation and electrical equipment reduced the net monthly import of electricity, the effects of such a measure with the absorption cooling system were the opposite. However, the issue of an optimum balance between export and import of electricity can only be addressed through an economic assessment, which is not within the scope of this work. The scenarios adopting more efficient absorption cooling showed a stronger sensitivity to the electrical utility efficiency.

The thermal-load-following CHP model was found to be superior to the other previously studied model from the first-law thermodynamic standpoint. The monthly average CHP efficiency of this model was higher and comparatively much less sensitive to seasonal variations. The thermal-load-following model offered a higher overall system efficiency (fuel utilization) as well.

Units: Dual

Citation: ASHRAE Transactions, vol. 110, pt. 2