DEVELOPMENT OF A SIMPLIFIED MODEL FOR ANALYZING A HEATING SYSTEM WITH STES AND INVESTIGATION INTO THE EFFECTS OF COMPONENT EFFICIENCY ON ECONOMIC AND ENVIRONMENTAL ASPECTS

The use of intermittent thermal energy sources, in combination with seasonal thermal energy storage (STES), may offer competitive heating solutions compared to conventional heating systems. Currently, influence of subsystem characteristics on overall system economic and environment aspects is not fully understood. In this study, the effects of consumer load, subsystem efficiency and thermal supply losses are investigated through parametric studies using a simplified model. The system considered includes a single residential building located in Ottawa, Ontario, with a solar thermal source, underground thermal energy storage unit and natural gas backup boiler. Overall the thermal supply piping and STES have the highest capital cost followed by the solar collectors and backup boiler. The consumer load has the greatest effect on economics and CO2 emissions. Due to high solar fraction the varying backup efficiency has little effect on system economics. INTRODUCTION The use of renewable energy resources, like solar radiation and wind, for heating is limited as they typically have intermittent production rates, which may not coincide with heating loads of a building (Pinnau and Breitkopf, 2015). Through the use of thermal energy storage (TES) the effect of source intermittency and fluctuating heat demand can be reduced (Kapsalaki et al., 2012; Pavlov and Olesen, 2012). Combining renewable energy resources with TES allows for energy to be collected and stored until there is a demand, which could lead to more economic utilization of intermittent renewable and alternative energy sources (Dincer and Rosen, 2011; Haeseldonckx et al., 2007; Kapsalaki et al., 2012; Pavlov and Olesen, 2011; Pavlov and Olesen, 2012). TES can be categorized by storage duration: short-, mediumand long-term. Short-term TES is used to assist in peak thermal loads and has storage lengths of hours up to a day (Sharma et al., 2009). Medium-term has a storage capacity from a day to a week. Long-term storage involves storage requirements from a week up to multiple months (Dincer and Rosen, 2011). Longterm thermal storage can take advantage of seasonal climatic variations and is often referred to as seasonal thermal energy storage (STES) (Dincer and Rosen, 2011; Gaine and Duffy, 2010; Novo et al., 2010; Schmidt et al., 2004). In northern climates, where seasonally varying space heating loads dictate energy consumption, STES can contribute significantly to intermittent thermal source integration for heat production (Dincer and Dost, 1996; Pavlov and Olesen, 2011; Novo et al., 2010). A common use for STES is the storage of heat in the summer for space heating applications in the winter (Sweet and McLeskey, 2012). Heating systems incorporating renewable energy sources, with STES, consist of five main subsystems: consumer, STES, intermittent/alternative source, backup source and thermal supply piping. The characteristics of each subsystem’s affect the operation of the heating system. Effective integration of STES into heating systems requires detailed knowledge of subsystem operation and understanding of the interactions between subsystems. Currently, limited work has been reported on the effects of intermittent/alternative and backup source characteristics, associated with a wide range of sources, on performance, economic and environmental aspects. Typically STES are custom built for the specific application and require detailed knowledge to conduct accurate performance, economic and environmental feasibility analyses. More information on the overall system design of heating systems with STES including the significance of subsystem parameters and interaction between subsystems would assist with the optimal integration into building applications and communities. Through investigating significant design considerations it may be possible to develop accurate guidelines for use in basic and detailed system designs. To help address these needs, a simplified method to analyzing the economic and environmental aspects of a heating system with STES is developed and presented here. In an attempt to quantify the significance of different subsystem parameters and the interaction between subsystems on system performance, parametric studies are conducted. The effects of varying consumer load, STES efficiency, intermittent source and backup source efficiency, supply network pipe losses on cost and CO2 emissions are investigated. METHODOLOGY A detailed thermodynamic analysis of heating systems with STES is not a significant focus of the present study. Rather, the influence of subsystem efficiency values and losses is of primary interest. Since a detailed thermodynamic analysis was not the focus of this investigation, estimation of the subsystem efficiency and losses were assumed to be within the range of typical values found in literature. The model is developed in such a fashion to allow for flexibility in analysis through independent variation in subsystem characteristics. Ideally, the interdependency of subsystem characteristics should be considered as variation of some characteristics may affect the choice of applicable subsystem technologies and range of operating conditions. Information about consumer heating load and intermittent source availability are key characteristics in the analysis of heating systems with STES. RETScreen® (Natural Resources Canada, 2016) was used to determine the consumer load characteristics over the course of a year using the combined heat and power project template contained in the software. System location, building floor area, and peak heating load of the building per unit of floor area were provided to the software. Values for total amount of thermal energy consumed annually, average rate of thermal energy consumption for each month and the peak rate of thermal energy consumption for the entire building space were returned. Ideally consumer load characteristics would be determined using consumer load profiles developed through daily, hourly, or minute by minute values. This method provides a simplified approach to estimate consumer heating load characteristics using monthly averages. This limits the accuracy of the study since the heating load may vary considerable from the average values provided by the software. Solar energy was taken as the intermittent source of heat production through the use of solar thermal collectors. RETScreen was used to determine the amount of thermal energy available for heat production using the solar power project template. System location, collector slope and solar azimuth were specified in the software and values of monthly average daily solar radiation per unit area on horizontal and tilted orientations were returned. The values were used for the thermodynamic analysis of the system. SYSTEM DESCRIPTION The proposed system model includes the main subsystems required for a heating system with STES including: consumer, intermittent energy source, STES, backup energy source and thermal supply piping. As a simplification the model excludes auxiliary components typically found in heating systems such as heat pumps, pumps, heat exchangers, controls and monitoring devices. Exclusion of these auxiliary components may affect the accuracy of the results as the energy losses and consumption associated with their operation are excluded in the analysis. The system considered is shown in Figure 1. The energy is supplied to the consumer through an intermittent source supply chain and a backup source supply chain. In the intermittent source supply chain energy is introduced to the system through the intermittent subsystem. The energy introduced to the system represents the amount of energy presented from the original intermittent energy source (amount of available sun). Heat is generated and transferred to the system through the intermittent source heat production subsystem and then split into two paths through pipe sections one and two. Pipe section one leads to the consumer for immediate use of the thermal energy. Whether the energy can be used immediately is dependent on the intermittent source production and consumer loads coinciding. When the rate of heat introduced to the system, by the intermittent source, exceeds the consumer load, it is directed through pipe section two delivered to the STES subsystem. Heat is extracted from the STES and delivered to the consumer through pipe section three when the consumer load is higher than the supply through pipe section one. A backup thermal energy source is included in the system to respond to the peak heating demands of the consumer and is contained in the backup source supply chain. Fuel is consumed (e.g. fossil fuel and electricity) by the backup subsystem and the thermal energy produced is delivered to the consumer through pipe section four. ANALYSIS The following outlines the assumptions, equations and other relations and expressions used perform a thermodynamic, economic and environmental analysis of the system in Figure 1. The following assumptions are invoked to simplify the analysis and to assist in developing the governing relations:  At the end of every annual cycle the amount of energy stored in the STES system returns the equivalent amount stored at beginning of the cycle (no annual thermal accumulation).  Heat transfer between thermal supply pipes and other components is considered adiabatic.  Intermittent source, backup source and STES efficiency are independent of subsystem capacity.  Losses per unit length of thermal supply pipe is independent of pipe capacity.  Intermittent source, backup source and STES specific cost are independent of subsystem capacity.  Energy quality is not considered; working temperatures are assumed to be correct for the transfer and use of energy in all subsystems.  Cost of thermal supply pipe per meter length is independent of pipe capacity.  Costs associated with maintenance, personnel, and decommissioning or refurbis