Simulation and exergetic analysis of renewable multi-generation units for a building group


Motivation
By the year 2050 the European Union has set a target to reduce greenhouse gas emissions by 80-95% with respect to emissions in 1990. To achieve these goals, all energy uses must be considered. District heating systems account for approximately 40% of worldwide energy consumption. District heating networks are currently powered by fossil fuels with high exergy values compared to the exergy values needed by consumers. Exergy analyses can provide an understanding of changes that can be made to the system for improved efficiency.
Exergy is best understood as the product of energy and energy quality and can be considered as the ability of energy to do work. The higher the temperature of a heat flow above the temperature of the surroundings, the higher the energy quality. Throughout this paper lower temperatures will be associated with low exergy and low energy quality. This assumption can be used to optimize the exergy efficiency of district heating systems. This is called the low exergy (LowEx) approach. The Low Ex approach allows for better matching of energy quality levels in order to optimize the utilization of high quality energy sources and minimizing of internal losses.

Scope
The purpose of this study is to investigate the possibility of improved exergetic efficiencies through the combination of various renewable energy supply technologies. The model includes ten residential buildings with high energy standards, a connected low-temperature heating network with substations, a buffer memory, and the heat generation units: solar thermal (ST) ,  heat pump (HP) with borehole heat exchangers, and gas-fired combined heat and power (CHP). There is additional storage and a control system connected to a generator to guarantee uninterrupted heat supply. The 35 m3 buffer storage tank is also applied to the system when the system utilizes HP or SHP.
The target supply temperature is 45°C. Ideally, every building will be able to receive this temperature from the district supply, but in the event that the target temperature is not achieved, each building also has an instantaneous electric water heater.
The data produced by the TRNSYS simulations performed on the models is then used in calculations looking at energy efficiency, exergy efficiency, and exergy consumption. A simplified economic evaluation is also used to compare the different supply models.

Results
From an exergetic point of view, solar heating is the most attractive heat source. However, there are large limitations: the heat is mostly produced during the summer, and must be stored with high heat losses until the heating season. This results in high costs that may not be economically viable. Because of this, it becomes necessary to also use a second heat generation unit. CHP systems are not a good option for combination as they are most efficient (monetarily and energetically) with high operating times. Using both an ST and an HP is the best combination. Exergy advantages provided by the ST can be combined with the more operation HP in the winter. A combination of HP and CHP is also an option, as there were trends for exergetic and economic efficiencies.

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