Energy supply system for a high-rise building in Germany : Optimization of a heating and cooling supply system made with plant simulations

Abstract: The building sector has great potential for energy savings since it is responsible for nearly a third of the global energy use. As oil, coal and natural gas continues to hold a big share of the energy mix it is important to work towards introducing more renewable energy alternatives. For an energy supply system, it is common to use a multi-energy system to increase efficiency. By increasing efficiency and the use of renewable energies, the total energy consumption can be lowered, and emissions can be reduced.   The purpose of this thesis was to design a multi-energy supply system for a high-rise building in Germany with low emissions. This was executed by constructing a simplified design used as a reference, also referred to as option 1, and two other designs with different energy sources, called design 1 and design 2. The design layout was constructed in collaboration with Transsolar. Design 1 is using a compression chiller and is presented as three different variants, option 2, 3 and 4. The three options were created to evaluate the pros and cons of a larger cold water storage tank and sizing of the compression chiller. Design 2, option 5, is using an absorption chiller and this option was created to investigate the opportunity to use district heating. Simulations were made of the designs with the simulation tool TRNSYS and presented in graph form. Values provided from the simulations were then used to calculate emissions, investment costs and net present value over a period of 40 years. For the net present value, three different variants were presented. One without carbon costs taken into consideration, one with low carbon costs and one with high carbon costs. The result was evaluated, and comparisons were made to suggest the most sustainable option for the building.   The result of design 1, which has a natural gas boiler for peak heating demand and a compression chiller as cold energy source, shows that the chiller produces a higher capacity compared to the installed value during the summer and a lower value during the winter. The norm capacity of the chiller in design 1 was 175 kW when combined with a larger cold water storage and 410 kW with no cold water storage. Comparison of those options showed that it is economical to install a cold water storage since the investment cost of the chiller is lowered. However, it also showed that a higher capacity of the chiller lowered the need for the gas boiler, because of the parallel heating and cooling characteristic of the chiller, which reduces emissions. Another option was made with a larger cold water storage implemented and an over dimensioned capacity of 340 kW for the chiller to reduce the gas needed for heating. For the option with the 175 kW compression chiller the gas boiler needed to supply 15% of the heating load of the building, while the option with the 340 kW compression chiller needed 3% of the buildings heating load to be provided by the gas boiler. This decrease in the gas boiler dependency reduced the emissions for heating from 52 ton/year to 41 ton/year, while still having comparable cost over time with low carbon costs and lower cost with high carbon costs. Therefore, the option with an over dimensioned chiller with an implemented big cold water storage is the favourable one, since it has the lowest emissions of the options of design 1 and is economically justifiable.   Design 2 has one of lowest investment cost and the lowest emissions which was 61% lower than the reference design and 19% lower than design 1. With higher carbon costs option 5 is the cheapest, otherwise it is relatively similar to the other options. Hence, Design 2 is the system that is suggested to use for the building.