Compare total costs of cogeneration-system alternatives

Today's emphasis on industrial cogeneration calls for a method of reasonably estimating costs of the many system alternatives. The approach differs from utility cost-estimating, mainly because steam capacity and power-generation capability are separate design objectives. Either the industrial powerplant meets the process-steam demand and then generates whatever power that creates, or else it meets the electric-power demand and generates the required steam. Choice of approach depends on the steam and electric-power requirements of the facility. Ideally, they balance exactly. In practice, most steam requirements will not generate enough electric power to meet the plant load. Conversely, the steam flow can rarely generate more electric power than the plant needs. Variations in steam conditions and turbine-exhaust pressure lead to many ways of matching the loads. More important, regulated buyback of excess electric power by public utilities now eases the problem of load balancing. Still, a profile of steam and electricpower consumption is necessary to begin. Daily, weekly, monthly, and seasonal variations are all important. During initial evaluation of the balance, use an average of 25 lb of steam/kWh as the steam rate of a small steam turbine. It is a conservative number, and the actual value will probably be lower--meaning moremore » electric power for the steam flow-but it will give a rough idea of how close the two demands will match. In this paper, capital costs are separated into costs for the boiler island and costs for the turbine island. Absolute accuracy is to within + or - 25%, not of appropriation quality but good enough to compare different plant designs. In fact, the relative accuracy is closer to + or - 10%.« less