Modelling Processes and Methods

In the review of current fuel cell micro-CHP models in literature there were found two approaches:

Firstly, detailed fuel cell and sub-component models which may include thermody- namics and electrochemical kinetics or other characteristics but neglect the building that the fuel cell micro-CHP will be installed into and its energy demand. The main goal is usually the calculation or maximisation of the electrical eciency or the power delivered by the fuel cell. Studies that have selected this approach are included in references [13, 20, 97].

Arsalis et al. have developed a model for a residential micro-CHP system based on a PEMFC. The system covers electricity, DHW and space heating for a home in Denmark. A detailed fuel cell micro-CHP system is modelled . However the house side of the system with its varying electrical and heating demand proles is not equally considered but has been simplied into three time periods: winter, summer, and spring (autumn) [13]. Palazzi et al. have implemented a techno-economic model of a SOFC micro-CHP. They formulated the model as an MINLP problem assigning dierent fuel processing options to binary variables. Their model can identify the optimum solutions for system eciency and specic investment cost [97].

Secondly, there are fuel cell micro-CHP models which consider the interaction of the energy plant with the building and its energy characteristics. Usually each researcher would dene a typical house that will be determine the energy demand; a denition which varies from study to study. They often use real data taken from eld studies or use Building Modelling Software to generate their own. The most popular software for building modelling is TRNSYS, ESP-R, IES and TAS [58, 87, 116, 119]. In some cases researchers obtain data of energy demand from actual or simulated dwellings and use them to determine the capacities and operating patterns of fuel cell systems. These models describe the system more accurately without only focusing on the fuel cell system. Studies that have followed this approach are included in [21, 73, 95]. Hawkes et al. in order to investigate the impact of the house demand prole, ex- amined patterns of heat demand that favour SOFC based micro-CHP. They looked at dierent heat demand proles for a UK dwelling with their model and performed a techno-economic analysis. A similar study was performed by Barelli et al. who developed a residential micro-CHP model consisting of the fuel cell, the required balance of plant and an auxiliary hot water boiler. The purpose of the study was the evaluation of the performance of fuel cell based CHP systems under variable electrical and thermal loads. Oh et al. performed an economic analysis of a sys-

tem which has its capital cost covered by government funding. They performed an optimisation study for a 1 kW PEMFC micro-CHP system achieving up to 20% sav- ings in the operational cost of the PEMFC-based CHP system, when the installation is covered by the government [95]. Other researchers have applied their models to systems located in other climates where heat demand is dierent and cooling might also be required. Ashari et al. presented a techno-economic study of a PEMFC fuel cell power CHP system designed for a residential building located in Tehran. They looked at the variation of the operating conditions of the system in relation to the electricity cost [16].

Studies that use dwelling data together with fuel cell micro-CHPs sometimes also investigate policy requirements that would allow the fuel cell micro-CHP market to grow. Pellegrino et al. investigated the technical and policy aspects of the fuel cell micro-CHP residential market, evaluating combinations of plant and operating modes together with various support schemes. They concluded that dwellings with high energy consumption would benet more from support schemes such as the feed- in tari [99].

Another trend among researchers seems to be the comparative analysis of various micro-CHP technologies suitable for domestic applications. They usually examine the feasibility of technologies which are already available in the market for micro- generation in dwellings. Publications on this topic are included in references [19, 77, 117].

The coupling of thermal storage tanks (TST) with micro-CHP plants is a common approach, so many researchers have modelled such systems. Most studies identify the optimum size of the storage tank for dirent cases and constraints. The constraints vary and could be space or cost limitations. Publications that focus on the eect of thermal storage are included in references [17], [24], [106].

Other researchers have also looked at the eects of plant capacity and control methods on system performance [64, 69]. A study that moves one step forward in terms of the involvement of the dwelling side of the fuel cell micro-CHP system was conducted by Gandiglio et al. [68]. They have modelled a 1 kWe PEMFC based micro-CHP system together with the balance of plant, coupled with an underoor heating system. However, even though the heating system is considered, no system sizing is attempted as the study is based on a 1 kWe fuel cell unit.

Some studies are based around the attempt of integration of dierent plant and equipment that can be used with a micro-CHP system. Such studies can be found in references [98, 100, 101, 114].

A review paper that provides useful information on fuel cell and optimisation has been prepared by Ang et al. [10]. It reviews the modelling advances in fuel cell systems and is not limited to stationary micro-CHP applications. It also looks at the models that have been developed for portable, stationary and transportation applications using fuel cells.