Choosing the Right Technologies – A Model for Cost Optimized Design of a Renewable Supply System for Residential Zero Energy Buildings

This work presents a methodology to identify and investigate the cost optimal design of supply systems for Low and Net Zero Energy Buildings with the focus on residential single family houses. A preliminary analysis investigating relevant literature and existing computer tools resulted in the conclusion that for this specific scope a lack of adequate approaches existed. It is briefly discussed, why linear programming and the software platform GAMS have been chosen to define the optimization problem. In the following the concept and structure of the methodology is explained with the main required input data and resulting outcomes. The theoretical part describes the general equations of the approach and summarizes the parameter set, which is applied to model individual technologies.

The next chapter describes each considered supply option in detail with relevant cost and technical data. Further, individual performance models are defined. For small scale residential systems the hot water tank is one of the main components, connecting supply and demand side and acting as a buffer during mismatch periods. For this reason, the developed hot water tank model is rather detailed accounting for three different temperature layers, two different supply and demand loops as well as individual heat losses. It is presented at the end of the technology chapter. Subsequently, the methodology is validated by investigating the output with one single technology at a time and thus the individual performance models in a case study. It is found that resulting energy generation rates are in reasonable ranges and according to the performance models defined throughout this work.

As an alternative option to validate the proposed methodology an experimental setup has been designed as a part of this work, which features most of the investigated technologies and would allow the comparison with performance under real conditions and different consumption patterns. However, the setup could not be taken into operation within the timeframe of this thesis but is briefly described in Chapter 5. This is followed by a summary of the main findings obtained during different case studies and which have been published and presented as a part of this thesis. It was concluded that lowering the 100% Net ZEB demand would lead to a reduction of overall system costs and that local optimal system solutions do not necessarily contradict with public grid interests, when the control is adapted. Further, results show that the assumption of constant efficiencies for CHP technologies does not lead to large changes in investment decisions.

However, considering flexible efficiencies might be important when optimizing operational schedules of an existing system involving fuel cells as their part load operation played a moderate role in the optimal solution of the conducted case studies. Additionally, a method is presented, which accounts for uncertainties in user behavior and weather profiles.