Importance of demand side unsteadiness on opportunity for space heating energy saving

Abstract

Much effort is focused on improving space heating system efficiency through improving technologies such as boilers and heat pumps, improving building energy losses through, for example, improved insulation and reducing wasted heating through improved building controls. The importance of the hydronic circuit in affecting the amount of wasted energy is often overlooked, in part due to the conscious and unconscious application of quasi-steady assumptions to energy analyses of buildings. Occupant behaviour, equipment use and solar irradiance are three examples which cause the demand on the heating system to be significantly unsteady despite stable room temperature requirements and relatively high building fabric thermal masses. This paper presents results from a single room validated unsteady space heating energy model used to explore the effects of demand side unsteadiness on energy consumption for heating system with on-off and more complex proportional-integral type controls. Demand side unsteadiness is shown to have a significant influence on the heating energy consumed, typically increasing it compared to a steady equivalent. With on-off thermostat control, the behaviour is highly non-linear, depending significantly on the timing of demand perturbances relative to 'on' events. The heating energy used exceeded the minimum required to maintain 20°C in cold and mild heating conditions by 1-14% and 14-75% respectively. With more complex proportional-integral based controllers, the excessive energy used exceeded the minimum required by 4-12% and 9-44% in cold and mild conditions respectively. High frequency demand side perturbations are effectively filtered out by the space's thermal mass. Lower frequency perturbations such as those arising from solar cycles are filtered out by the heating system control response. However, there remains frequencies of perturbance which are similar to the hydronic system response timescales which are not well managed, leading to the excess energy provision.