Applying Relative Net Present or Relative Net Future Worth Benefit and exergy efficiency for optimum selection of a natural gas engine based CCHP system for a hotel building

Multi-objective optimization of a natural gas engine (NGE) based combined cooling, heating and power (CCHP) system for selecting the number and nominal power (NP) of NGE(s), their partial load (PL) during a year (operation strategy), heating capacity of auxiliary boiler as well as cooling capacities of absorption and electrical chillers are performed in this paper. Two objective functions were either Relative Net Present Worth Benefit (RNPWB) or Relative Net Future Worth Benefit (RNFWB) and exergy efficiency. The selection procedure was performed in two selling (excess electricity generated can be sold to the grid) and no selling (excess electricity generated cannot be sold to the grid) modes. For our case study two NGEs (with non-similar nominal powers of 1050 and 1350 kW) in selling mode and one NGE (with nominal power of 2060 kW) in no selling mode were optimum results by using either RNPWB or RNFWB and exergy efficiency. Furthermore in selling mode, with forcing a constraint (two similar NGEs), two 1100 kW NGEs were selected. Moreover by forcing another constraint (selecting one NGE) a 2300 kW NGE was selected. Results showed that selecting two non-similar NGEs (without forcing any constraint), had payback period 4.1 years. This case provided the maximum RNPWB (1013.5 × 104 $) or RNFWB 4233.6 × 104 $) and exergy efficiency (41.16%). Results of sensitivity analysis for one NGE based CCHP system with increasing equipment and unit energy costs were also reported.