Combined heat and power based district energy systems increase energy efficiency in remote communities
Many remote communities in Canada currently rely on diesel generators for meeting their electricity needs. Aside from being costly to operate due to high fuel requirements, diesel generators are also large emitters of CO2. Some communities have managed to displace diesel generation by including renewable energy technologies (e.g. wind-diesel systems) into their energy mix. Although effective in lowering emissions and reducing fuel consumption, these integrated systems only affect part of the whole energy picture; electricity consumption. A large proportion of a community’s total energy demand (approximately 80%) is attributed to residential domestic hot water and space heating requirements. Generally, these heating loads are met separately, building by building, via independent oil or propane fuelled (and in some cases electric) heating systems.
A District Energy (DE) system is an integrated system in which energy (in the form of steam or hot water) is generated locally and is distributed to individual buildings within a piping network. High energy efficiencies can be achieved in a DE system as heating loads are aggregated and managed simultaneously through a centralized plant. A DE system is an attractive alternative for a remote community due to the potential for waste heat recovery from the diesel generator plant; with proper retrofit, the diesel generator plant is transformed from a power only plant to a Combined Heat and Power (CHP) plant. Diesel fuelled CHP-based DE systems have been implemented in several off-grid Canadian communities to date such as Fort McPherson, Cambridge Bay, Igloolik, and Iqaluit, and are shown to be feasible from both a technical and economic standpoint. A DE system, designed for operation in a remote community, may include but is not limited to the components shown in Figure 1. Figure 1 shows a system comprised of a diesel CHP plant; a biomass CHP plant; a biomass boiler plant; a hot water storage tank; and the following Renewable Energy (RE) generators: Solar PV, Wind, Micro-hydro, and Wave. The energy system depicted in Figure 1 operates as follows: Energy is recovered first by the diesel CHP plant; the recovered heat is transmitted to a hot water storage buffer tank located in or adjacent to the diesel generation facility. A biomass CHP and/or boiler plant, either located at a substation some distance away (closer to the primary heating loads) or at the diesel generation facility provides additional energy to the DE system. The DE system delivers a maximum of 40% to 70% of peak heating load to the buildings connected to the network; the rest is provided by auxiliary in-building heating systems (e.g. an oil furnace for a typical building in Figure 1). Hot water in the DE system is circulated via variable speed pumps. Automated valves at each building respond to calls for heat from each building’s thermostat. Balancing valves ensure an even supply of energy throughout the system. To meet the space heating load, hot water supplied from the DE system flows through a fan coil unit which is coupled to the existing building forced air furnace. To meet the domestic hot water load, a small heat exchanger is used to bring the hot water tank temperature to its desired set point.
Figure 1 also shows PV, wind, micro-hydro, and wind generators as potential sources of electrical energy for the community. Increasing the RE penetration in the electrical grid directly results in a reduction in diesel fuel consumption. However, as diesel generators (in the absence of a battery-inverter control system) are solely tasked with the responsibility of maintaining grid frequency and stability, instantaneous RE penetrations are not permitted to be greater than approximately 30%. Any power produced by RE generators exceeding this limit is sent to a useful thermal dump load; the dump load is shown in Figure 1 as a resistor located in the hot water storage tank. Exceptions apply however with biomass CHP. With the establishment of biomass as a viable energy resource in the community, biomass CHP can be implemented seamlessly while taking advantage of local RE fuel sources, and labour resources. Biomass CHP can displace a significant portion of diesel generated electricity, providing advantages in terms of electrical grid stability, thermal energy supply, and diesel fuel reduction.
Research is currently underway to assess the impacts from integrating CHP-based DE systems in remote communities. A time series simulation model has been constructed in Matlab/Simulink. Minute time steps are used in the simulations for the duration of one year. Potential reductions in fuel utilization and CO2 emissions (relative to the diesel stand-alone reference case) are quantified for a grid comprising a diesel CHP plant and a wind turbine. A central boiler and heat storage tank are also considered in the analysis. The community of Hesquiaht, located on the west coast of Vancouver Island in British Columbia is used as the case study. Results show that reductions in fuel utilization and CO2 emissions of up to 55% are possible relative to the reference case when switching to a system encompassing wind-district energy infrastructure. Future work will assess the feasibility of using other RE generator types such as wave energy converters in remote CHP-based DE systems.