Energy analysis of a novel domestic scale integrated wooden pellet-fueled micro-cogeneration concept
Introduction
The most common domestic scale micro-cogeneration technologies within the power range of 0–2 kWe are fuel cells (FC), Stirling engines (SE), Organic Rankine Cycle (ORC) and internal combustion engines (ICE), the most common nominal electrical power being 1 kWe in commercial products [1]. In practice, the internal combustion engine (ICE) is the only time-tested technology with an established market [2]. About 30,000 micro-CHP units based on 5 kWe ICE's have been installed in Germany in the recent years [3]. Average thermal efficiencies of 76% and electrical efficiencies of around 12% have been achieved. With the fuel cells, a high electrical efficiency (up to 40% LHV) and a favorable electrical to thermal ratio (ETTR) (up to 0.59) can be obtained [1]. However, the market is restrained by the emerging market status of fuel cell technology. The need of fuel processing (e.g. gasification, reformation or electrolysis) can be regarded as a disadvantage in terms of market development. Stirling engines (SE) raise to the challenge with their ability to utilize a variety of thermal sources. The technology is still emerging, but widely cited in the context of domestic scale micro-cogeneration [4].
Requirements related to flexible thermal sources, efficiency, operational temperatures, controllability, noise, space requirement and capital cost have increased the willingness to introduce new micro-cogeneration technologies. According to Dong et al. [5], the market potential of biomass-fueled micro-cogeneration technology has been recognized worldwide, but the level of its commercialization and implementation is still low. Bonnet et al. [6] suggest the Ericsson engine as an alternative to SE due to its less complicated design and thus potentially reduced costs. Organic Rankine Cycle (ORC) has been considered a promising, low-cost alternative for biomass-fueled micro-cogeneration, capable of operating at low temperatures, which makes it suitable for waste heat recovery [7].
A rotary steam engine (RSE) is an oil-free medium-speed (1000–2000 rpm) engine. It is capable of exploiting versatile thermal sources and steam temperatures of 150 to 180 °C, which allow operational pressures less than 10 bar for electrical power ranges of 1–20 kWe. The concept would be simple, small and quiet option for micro-cogeneration. An RSE can be easily integrated in commercially available biomass-fired household boilers. Alanne et al. [8] conceptualized an RSE to be integrated with a commercial pellet-fueled boiler and modeled and analyzed the performance of the system.
A domestic thermoelectric cogeneration system (DTCS) is a noiseless and maintenance-free method of on-site micro-cogeneration, with no moving parts. Zheng et al. [9] developed a bench-scale experimental prototype of a DTCS to be integrated in a domestic boiler system. Their target was to improve the electrical efficiency by better organizing the heat transfer and to avoid the use of a cooling fan, which was mentioned as a disadvantage in the stove applications. Their study did not address the boiler operation according to the user's needs and boiler size. Alanne et al. [10] characterized and analyzed a concept where thermoelectric modules were directly integrated in the combustion chamber and convection tubes of a commercial wooden-pellet fueled boiler, to keep structural changes minimal and the temperature difference large.
The need for energy matching originates from the legislative challenge to meet the objective of nearly-net-zero-energy buildings by compensating the energy imported from the grids [11]. To assess the energy matching of individual buildings employing various on-site micro-cogeneration technologies, Cao et al. [12] suggested four energy matching indices. Two on-site energy fractions (OEFe and OEFh) indicate the proportion of the load covered by on-site generated electricity and heat, whereas the on-site energy matching indices (OEMe and OEFh) determine the proportion of on-site generated electricity and heat used in the load instead of dumping or exporting it. The index numbers vary between zero and unity and the higher the value is, the better is the applicability of the system in the examined building. With micro-cogeneration technologies, the averaged matching index AMI = 0.85 was acquired as a baseline of good matching, when an electrical storage is not used, and AMI = 0.75, when it is present [12].
The earlier work of Alanne et al. [8], [10] focused on assessing the saving potentials of the two new micro-cogeneration concepts in primary energy, carbon dioxide emissions and annual energy costs. Instead, they neglected assessing the impact of the building's energy performance level, electrical and thermal grid feed-in options, size of the on-site electrical storage, and energy matching. In the present study, the performance of the above micro-cogeneration systems are analyzed for two energy performance levels and three electrical demand profiles in three climatic zones in Finland.
Section snippets
Target building
The target building is a one-floor single-family house with the heated area of 134.3 m2. The house is equipped with a mechanical ventilation system and the heating system is programmed to maintain a room temperature of 21 °C. No cooling system is present, which allows the temperature to rise during summer months. The blueprint and a 3D model of the building are shown in Fig. 1.
Two energy performance levels are applied in the present study. The “1960 house” is represented by the default values of
Energy balances
The energy performance of the target building was investigated in three locations in Finland: Helsinki, Jyväskylä and Sodankylä. The hourly demands profiles (hourly average power) were calculated for two energy performance levels (1960 house and ULE) and each climate using the IDA-ICE whole-building simulation tool, which has been validated, for example, in Travesi et al. [23] and Loutzenhiser et al. [24]. In validity tests, the variation between actual energy demand and energy demand estimated
Conclusions
This paper evaluates the feasibility of conceptual, pellet-fueled micro-cogeneration technologies by way of annual energy balances and weighted energy matching indices (WMI). The concept is based on a commercial 20 kWth wooden-pellet fueled boiler. The study contributes to the research by evaluating the feasibility for both thermal and electrical tracking strategies and examining the option to charge an electric car and to deliver surplus heat to a local thermal grid in detached houses
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