Externally Fired Micro Gas Turbine and ORC Bottoming Cycle: Optimal Biomass/Natural Gas CHP Configuration for Residential Energy Demand

Small scale Combined Heat and Power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly true when biomass fuels are used. In most cases, the use of both heat and electricity to serve on site energy demand is a key issue to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and is influenced by climatic conditions, in particular in the case of residential and tertiary end users. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small scale micro turbine for heat and power cogeneration, a bottoming ORC system can be coupled to the cycle, however this option reduces the temperature and quantity of cogenerated heat available to the load. In this perspective, the paper presents the results of a thermo-economic analysis of small scale CHP plants composed by a micro gas turbine (MGT) and a bottoming Organic Rankine Cycle (ORC), serving a typical residential energy demand. For the topping cycle three different configurations are examined: 1) a simple recuperative micro gas turbine fuelled by natural gas (NG), 2) a dual fuel EFGT cycle, fuelled by biomass and natural gas (50% energy input) (DF) and 3) an externally fired gas turbine (EFGT) with direct combustion of biomass (B). The bottoming cycle is a simple saturated Rankine cycle with regeneration and no superheating. The ORC cycle and the fluid selection are optimized on the basis of the available exhaust gas temperature at the turbine exit. The research assesses the influence of the thermal energy demand typology (residential demand with cold, mild and hot climate conditions) and CHP plant operational strategies (baseload vs heat driven vs electricity driven operation mode) on the global energy efficiency and profitability of the following three configurations: A) MGT with cogeneration; B) MGT+ ORC without cogeneration; C) MGT+ORC with cogeneration. In all cases, a back-up boiler is assumed to match the heat demand of the load (fed by natural gas or biomass). The research explores the profitability of bottoming ORC in view of the following tradeoffs: (i) lower energy conversion efficiency and higher investment cost of high biomass input rate with respect to natural gas; (ii) higher efficiency but higher costs and reduced heat available for cogeneration in the bottoming ORC; (ii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid.