Modelling the optimum thermal store and biomass boiler size for a small communal heating scheme

Abstract

This paper presents a study of biomass boiler and thermal store optimisation for a small communal heating scheme. Poor system sizing has been identified by the Carbon Trust and CIBSE as affecting the performance of biomass heating systems, and a review of installations under the Renewable Heat Incentive has shown that typically systems have low utilisation and are less efficient than expected. Thermal store sizing affects the boiler efficiency and a literature review has highlighted that there is little research directly addressing the subject of optimisation of a thermal store for biomass heating systems. This paper studied the problem using a transient simulation of a heating system to establish the boiler and thermal store size that gave minimum energy consumption and the optimum life cycle cost for the range of options. The model demonstrated that for the systems assessed, the smallest boiler that met the design day average load used the least energy and larger thermal stores achieved greater seasonal efficiency. However the Renewable Heat Incentive tariff structure encourages oversized boilers with low utilisation and this gives rise to low seasonal efficiencies. Practical application: This paper looks into the role of thermal stores in improving biomass boiler efficiency. It also discusses the impact on efficiency that results from decisions to oversize biomass boilers in an attempt to maximise Renewable Heat Incentive income. The conclusions will be relevant to designers of biomass systems who are weighing up the implications of different boiler outputs and heat stores with regard to overall system performance. It may also interest operators who are investigating system performance and policy makers who are looking at how the tariff system could be affecting boiler size and performance.