As part of the owner's commitment to reducing energy consumption, all substantial projects require an extensive analysis of renewable energy and energy reduction systems. For this project, many different systems were evaluated, including: photovoltaic (PV), solar thermal, combined heat and power (CHP), biomass CHP, and dedicated heat recovery chiller.
This brief discussion of the systems, evaluated in general terms, provides a basis for comparison that can be applied to other projects. In the analysis performed for the owner, more detailed calculations of these systems were performed using energy modeling software. However, this simplified analysis can identify opportunities where a more detailed analysis is appropriate.
Methodology
The general methodology was similar for all of the systems evaluated. Annual energy costs and savings were calculated for each system and compared to the costs of installing and maintaining the systems. For the owner, a net present value (NPV) analysis was used to determine which options provided the best NPV. Here, a simple payback method is used to evaluate the options.
For consistency, units of energy have been converted to therms. A therm is a unit of energy equal to 100,000 BTUs or 29.3 kWh.
Utility costs for the St. Louis market, for example, are relatively low. The average annual electricity cost is only $0.06 per kWh or $1.76 per therm. The owner has contracts to purchase natural gas at market rates, so the utility costs for natural gas are generally low but can be very volatile. In the previous seven years, the average cost for natural gas was $0.80 per therm. For the purposes of this analysis, this average cost was used.
The new central plant for the campus includes boilers that produce 100 PSI steam for distribution to the campus. With flue gas and blow-down energy recovery, the boiler system has an annual average efficiency of 86%. This provides a base cost of steam of $0.93 per therm.
