The primary goal of the RETScreen Combined Heat and Power (CHP) model is to calculate the amount of energy delivered, in various forms, by an energy system. More specifically, a CHP system can satisfy heating, cooling and power loads by using a combination of base, intermediate and peak power, cooling and heating systems (See Figure 3) using various fuels and operating modes. The challenges of the model are therefore to:

1. assess the system’s energy needs in terms of heating, cooling and power generation;

2. estimate how those needs can be met by the various energy systems that are ultimately chosen.

Consequently, the model is devoted to calculating the system’s load and energy use (see note 6) and to evaluating how they can be met. The algorithm used by the model is schematically shown in Figure 3.

The various sections of this chapter each deal with a particular aspect of the calculation. Section ‎2.1 explains how heating load and energy use are calculated; section 2.2 considers the same calculation for cooling load and energy use, and section 2.3 addresses the calculation of power generation load and energy use. Section 2.4 gives a detailed description of the combined heat and power equipment that can be used. Finally, section 2.5 provides an overview of the possible operating strategies and calculates the energy delivered by the base, intermediate and peak systems.

Section ‎2.6 deals with calculations not covered in the previous sections, such as correlation formulae to estimate building loads or the heating value of fuels. Validation studies, summarized in sections ‎2.7 and ‎2.8, describe how the model was validated against other models and experimental data.

6. Load refers to instantaneous values (power, expressed in W for example), whereas energy use or needs refers to integrated values (energy, expressed in J or in Wh for example). For example, the heating load of a building refers to the heat that it requires at any given time, whereas its energy use or needs refers to the amount of energy it requires over a month or a year. Both are of interest for energy calculations: knowing the load is necessary to properly size the equipment, and knowing the energy use is necessary to evaluate the annual fuel consumption. Loads will be referred to by the symbol P throughout this chapter; energy use or needs will be referred to by the symbol Q. Average monthly loads will be referred to by the symbol .