Small CHP Research Articles

Energy efficiency analysis of Organic Rankine Cycles with scroll expanders for cogenerative applications

Small scale Organic Rankine Cycle (ORC) systems has been the object of a large number of studies in the last decade, because of their suitability for energy recovery and cogenerative applications.The paper presents an ORC numerical model and its applications to two different case studies; the code has been obtained by combining a one-dimensional model of a scroll machine and a thermodynamic model of a whole ORC system. Series production components, such as scroll compressors, from HVAC field, have been first considered in order to reduce costs, because this is a critical issue for small scale energy recovery and cogeneration systems. The detailed model of the scroll machine is capable to calculate the performances of both a compressor and an expander, as function of the geometry of the device and of the working fluid. The model has been first tested and validated by comparing its outputs with experimental tests on a commercial scroll compressor, then used to calculate the working curves of commercial scroll machines originally designed as compressors in the HVAC field, but operating as expanders. The model of the expander has been then integrated in the thermodynamic model of the ORC system.A series of comparisons have been carried out in order to evaluate how the performances are influenced by cycle parameters, scroll geometry and working fluid for different applications. The results confirm the feasibility of small scale CHP systems with acceptable electrical efficiency, taking into account the low-temperature thermal source, the small power output and the low-cost series production components employed.

Emission Calculation Methodologies for CHP Plants

The paperdiscussesdifferent calculation methods to correctly express the emission level of air pollutants produced byCHP(CombinedHeatandPower) systems, in order to take into account the environmentalbenefitdueto cogeneration. New and established methods to estimate the emission saving due to cogeneration of heat and power are reviewed and compared, with reference to small scale CHP systems; the paper clarifies that a proper emission assessment of CHP systems should consider both the global and the local scales of environmental impact. In particular, the method of the “avoided heat generator” is proposed in this study, for a local-scale environmental impact evaluation. This approach calculates the reduction of emission due to CHP operation, in comparison with the non-CHP operation of the same machine, taking into account the amount of pollutant emitted by a heat generator which provides the same thermal power of the cogenerator. Moreover, it is shown in the paper that another method, based on the PSI (Pollutant Saving Index) value, is suitable to estimate the global-scale environmental impact. The problem of the most representative unit of measure is also discussed, highlighting the advantages of an “output based” approach and providing easy-to-use formulas and graphs for the conversion of concentration values into output-based emission factors. Finally, a numerical evaluation of the CHP environmental benefit is provided for small electric power size CHP machines.

Engineering analysis of uncertainties in the performance evaluation of CHP systems

From January 1, 2011, in all UE countries the combined production of electric (or mechanical) and thermal energy (also called Combined Heat and Power, CHP, or cogeneration) is recognized as a high efficiency technology only when it is able to ensure a minimum value of energy saving with respect to the separate production of the same energy flows. The Directive 8/2004/EC, and a few successive Decisions of the European Commission, introduced a methodology to establish whether any cogeneration plant, existing or new, can be acknowledged as a high-efficiency CHP plant, and can therefore be supported from the UE member states. In the paper, such methodology, based on the evaluation of a standard Primary Energy Saving (PES) index, is briefly described, and then a metrological analysis is presented, in order to evaluate the uncertainties affecting the field evaluation of such index. Three numerical examples are also presented and discussed, referred to natural gas plants, showing that the evaluation of the PES index can be quite critical, especially for values close to the minimum limit fixed by the Directive, and in particular for small and medium scale CHP units, mainly due to the low accuracy that usually affect, in such cases, the measures of the fuel input.

SOFC Power Generation from Biogas: Improved System Efficiency with Combined Dry and Steam Reforming

Solid oxide fuel cell systems (SOFC) can improve the efficiency of electrical power generation from biogas compared to small scale CHP units. The SOFC requires a reforming step converting biogas into synthesis gas (hydrogen and carbon monoxide). . Dry reforming of methane with the carbon dioxide contained in biogas is a promising approach to produce high caloric SOFC fuel gas, but carbon formation is predicted for all typical biogas compositions. To avoid carbon formation and to compensate fluctuating biogas compositions additional supply of steam is essential leading to a combined dry and steam reforming. The paper describes the system approach, upfront analysis, biogas monitoring results, process unit design and evaluation as well as the system setup to demonstrate a biogas to electricity route with a gross efficiency of 45 % using a commercial 1 kW SOFC stack.

A thermodynamic criterion for selection of working fluid for subcritical and supercritical domestic micro CHP

A thermodynamic criterion for selection of a fluid both for subcritical and supercritical organic Rankine cycle has been proposed. Theoretical performances of few fluids have been comparatively assessed for use in low-temperature domestic organic Rankine cycle micro systems. Of the 20 fluids investigated, ethanol, R123 and R141b appear as the most suitable for small scale domestic CHP applications.

Small stationary reformers for H 2 production from hydrocarbons

This paper describes the activities performed in ENEA (Italian National Agency for New Technologies, Energy and Environment) during the last years in order to investigate the hydrogen production from largely available hydrocarbons, such as natural gas [1] and LPG (Liquefied Petroleum Gas), aimed at feeding Polymer Electrolyte Fuel Cells (PEFC) in the 1–5 kW electric power range. Reformers in such a size will be used for small domestic CHP systems (residential, hotels, offices, shopping centres) and for APU on boats and caravans. A significant part of the problems related with residential CHP based on PEFC occurs in the fuel processor [2]: start-up time is to be reduced, efficiency and reliability are to be increased. For this purpose ENEA has tested two LPG fed small reformers with a hydrogen production capacity of about 1 Nm 3/h of H 2 equivalent, both based on the steam reforming process. The reformer test station was provided with instrumentation in order to evaluate the performances of the reformer in terms of H 2 yield, CO content of the syn-gas, thermal and energy balance. The experimental results show the feasibility of such a reformer and indicate the lines for further improvements. In particular the methanation process looks like more suitable than PSA for reformate gas purification in reformers of such a size. The quality of the reformate gas, in term of low CO content, is quite good for feeding PEMFC. The duration and the stability of the start-up time must be improved. The reformer efficiency range is 75–85%.

Potentials and feasibility assessment of small scale CHP plants fired by energy crops in Puglia region (Italy)

The proposed work aims to assess the energy crop suitability of Puglia Region (Southern Italy), the techno-economic feasibility of small scale CHP plants fired by energy crops, and the environmental performances of the proposed CHP plants. In the first part of the work, a GIS model is defined and applied to evaluate the land suitability for energy crops in the Puglia region. In the second part, a financial appraisal of small scale CHP plants under Italian legislative framework (feed in tariffs) is proposed; the two case studies of bio-oil fired internal combustion engine (ICE) coupled to vegetable oil mill plant and fed by oil seeds (brassica carinata seeds) and syngas fired engine coupled to a pellet production unit and fed by herbaceous energy crops bales (fibre sorghum) are investigated. In the third part, the energy balance and the CO2 emissions of the whole bioenergy routes are assessed, in order to calculate the costs for the community (in terms of subsidies) to save a tons oil equivalent (TOE) of primary energy and to avoid a tCO2 in the atmosphere by these small scale routes. The results report a potential in Puglia Region of about 293 and 729 kt y−1 of brassica carinata seeds and fibre sorghum bales, respectively; the financial appraisal of the proposed chains, under the Italian legislative framework, reports an internal rate of return (IRR) of 38% and 17%, respectively, while the energy balance assessment reports an overall efficiency of the bioenergy routes of 2.72 and 2.95, respectively.

Energy and environmental assessment of small district heating systems: Global and local effects in two case-studies

In this paper, starting from previous works, further analyses on district heating cogeneration systems are proposed and particular attention is given to the NO x pollutant. In addition to the mean yearly concentrations, the hourly limit values for the protection of human health are also taken into account: the 18th maximum hourly concentration is considered. The main steps necessary to characterize a district heating plant are shown and two case-studies are discussed. As far as the energetic aspects are concerned, these new plants always offer a primary energy saving, and the global environmental effects are improved due to a reduction in CO 2. Local NO x emissions of a future plant do not decrease with respect to the existing emissions, and this aspect is closely connected to the type of CHP unit that is adopted (in this paper, natural gas fuelled engines). The local environmental effects due to NO x are estimated using a dispersion model, and the NO x concentrations (annual mean value and maximum hourly value) introduced by the new plant and those avoided through the non-use of existing domestic boilers are compared. For one of the case-studies emerges that some zones do not satisfy ambient air quality limits. For these reasons, a higher stack has been assumed for this critical case and the new results show that a reduction in the frequency of the overcomings, which is in agreement with expectations, can thus be obtained. The analysed case-studies have shown that even a small CHP district heating plant can produce critical aspects and a pollutant concentration analysis is a useful tool to check local environmental aspects.

A comparison of Miller and Otto cycle natural gas engines for small scale CHP applications

This paper presents an investigation into the feasibility and potential advantages of a small scale Miller cycle natural gas engine for applications such as domestic combined heat and power systems. The Miller cycle engine is compared to a standard Otto cycle engine using cycle analyses and multidimensional simulation, and basic engine design implications are discussed. It is found that the Miller cycle engine has a potential for improved fuel efficiency, but at the cost of a reduced power to weight ratio. A fuel efficiency advantage of 5 → 10 % compared to a standard Otto cycle engine appears possible, however it is stated that further investigations, in particular into the topic of engine friction, are required in order to validate the findings.

Transient Modelling of a Rotary-Kiln Pyrolyser

The increasing interest in small-scale renewable-energy plants for distributed power generation has led to a demand for suitable software tools to study and develop control systems able to manage advanced integrated systems. Biomass, as a renewable energy resource, needs to be processed if it is to be exploited in small CHP units and pyrolysis is one of the options available for transforming solid biomass into useful liquid and gaseous fuels. This work is concerned with the development of a time-dependent model of a rotary-kiln pyrolyser for biomass: the model is intended for the de- velopment of control systems and the simulation of integrated energy systems, where the pyrolyser is connected to a power-generation package. The model was developed within the TRANSEO environment and based on a quasi 2-D nu- merical discretisation of the rotary kiln. Results are shown for a real case that is currently under construction: the model is able to predict the impact of different operating conditions on fuel yields, as well as capturing the main transient phenom- ena occurring during changes in the pyrolyser operating conditions. Kewords: Biomass, pyrolysis, distributed generation, transient modelling.

Economic appraisal of small biofuel fired CHP plants

A study is presented in which an optimisation model is built to determine the economic viability of three small biofuel fired CHP plants. The model formulated to describe heat and electricity production with CHP plants is based on simulation data for three existing plants. The modeled system extends from the production facilities to the distrcit heating substations of the consumers, thus combining operational and design planning. We find that, compared to the long term average situation in the Nordic countries, higher electricity prices or lower investment costs are needed to make CHP plants attractive in small district heating networks. The results are relatively sensitive to the original economic parameters, and this uncertainity over future cashflows may further reduce investments in small CHP plants.

Case analyses of heat trading between buildings connected by a district heating network

Abstract This study examined the decentralisation of energy production with possibilities of trading not only electricity but also heat. This was carried out with a limited population of buildings connected to the district heating network in a small town in Finland. The study examined whether a buildings community could be self sufficient with respect to heat by using small CHP plants, and explored the consequences of such an energy system. It also aimed to find an optimal solution. Therefore, it searched in what type of buildings should the micro-CHP plants be installed and with what kind of strategy should they be operated in order for the community to be energetically self sufficient without producing lots of excess heat.

05/02568 Optimal designs of small CHP plants in a market with fluctuating electricity prices

05/02568 Optimal designs of small CHP plants in a market with fluctuating electricity prices

Integrated distributed energy evaluation software (IDEAS): Simulation of a micro-turbine based CHP system

IDEAS was defined in University of Newcastle upon Tyne in United Kingdom as a joint academic-industry project, in order to develop a comprehensive software package for designing, optimizing and monitoring of distributed energy systems based on micro-turbine, fuel cell and internal combustion engine driven systems using fossil fuels and renewables. The IDEAS idea shaped in pursue of a series of actual projects at Research Centre for Innovation and Design (RCID) to suggest the most energy consumption optimized and environmentally friendly distributed CHP system for some new residential complex as well as some academic PhD thesis at Mechanical and Systems engineering department to optimize the performance of gas turbine and micro-turbine driven CHP system for generating power, cooling and heating. Increasing number of CHP equipment suppliers and consultant engineering companies through UK and Europe and approved inspiring short-term and long-term policies of EU for implementing the CHP systems, fascinating successful experiences in some EU countries like Denmark in CHP implementation and lack of a comprehensive European based software package for designing and optimizing of these systems, all supported the idea of developing the IDEAS. IDEAS tends to be a complement to tens of small CHP systems related European software, each has been developed with limited data and a European version of similar powerful non-European (mostly American) software packages. To gain an overview of the difficulties, musts, domain of the package abilities, required financial, human resources and information and also for a better presentation of the idea to absorb supports from both academic and industry possible partners, a micro-turbine driven CHP system was designed for a 71 dwelling residential complex in north of England. This paper presents the works which have been done and yielded results about the requirements of developing IDEAS.

Integrated energy systems and local energy markets

Significant benefits are connected with an increase in the flexibility of the Danish energy system. On the one hand, it is possible to benefit from trading electricity with neighbouring countries, and on the other, Denmark will be able to make better use of wind power and other types of renewable energy in the future. This paper presents the analysis of different ways of increasing flexibility in the Danish energy system by the use of local regulation mechanisms. This strategy is compared with the opposite extreme, i.e. trying to solve all balancing problems via electricity trade on the international market. The conclusion is that it is feasible for the Danish society to include the CHP plants in the balancing of fluctuating wind power. There are major advantages in equipping small CHP plants as well as the large CHP plants with heat pumps. By doing so, it will be possible to increase the share of wind power from the present 20 to 40% without causing significant problems of imbalance between electricity consumption and production. Investment in increased flexibility is in itself profitable. Furthermore, the feasibility of wind power is improved.

Small distributed generation versus centralised supply: a social cost–benefit analysis in the residential and service sectors

This paper aims at measuring the social benefits of small CHP distributed generation (DG) in the residential and service sectors. We do this by comparing the social costs of decentralised and centralised supplies, simulating “ideal” situations in which any source of allocative inefficiencies is eliminated. This comparison focuses on assessing internal and external costs. The internal costs are calculated by simulating the optimal prices of the electricity and gas inputs. The external costs are estimated by using and elaborating the results of the dissemination process of the ExternE project, one of the most recent and accurate methodologies in this field. The analysis takes into account the main sources of uncertainty about the parameter values, including uncertainty about external cost estimations. Despite these sources of uncertainty, the paper concludes that centralised supply is still preferable to small DG. In fact, the overall range of DG social competitiveness is restricted, even considering further remarkable improvements in DG electrical efficiency and investment costs. The results are particularly unfavourable for the residential sector, whereas, in the service sector, the performance of DG technologies is slightly better.

Potentials for electricity production from wood in Ireland

Greenhouse gas emissions in Ireland are expected to be 25% higher in 2010 as compared to 1990, whereas Ireland's Kyoto commitment is a maximum rise of 13%. One of the measure taken to reduce this gap (about 7 Mtonne CO 2-eq.) could be the development of biomass based electricity production. This study focusses on various biomass resources and a range of conversion technologies, in order to select promising routes for biomass based electricity in the short term. This article extends previous analyses by: (i) including the use of forest residues and sawmill residues as potential biofuels; (ii) considering both co-firing in retrofitted existing peat stations and co-firing in the new to be built Europeat power plant; and (iii) including other promising conversion technologies to generate electricity from biomass. Although the potential for energy crops in Ireland could be large, their cost is relatively high, 4.7–8.2 GJ −1, dependent on the type of land used. Lower cost biomass can be obtained from currently unutilised forest residues. It was estimated that about 3.4 PJ could be extracted from the forests at a cost of 2–3 GJ −1. The present market price of sawmill residues (in total about 7 PJ) is 1.4–3.5 GJ −1. Three promising routes for electricity generation were selected: co-firing with peat in the new to be built Europeat power plant, small scale CHP generation with fixed bed gasifiers, and co-firing in the Moneypoint coal fired plant. Co-firing in the Europeat plant is the most attractive. The electricity production costs are 0.033–0.053 kW −1 h −1 as compared to 0.041 kW −1 h −1 for peat based electricity. Costs per avoided tonne of CO 2 are between −7 and 12 . Electricity generation from wood could fill about 14% (about 1.0 Mtonne CO 2-eq.) of the Irish ‘Kyoto gap’.

Wind of change

Innovations in gas turbine technology are promising to transform the business case for small CHP schemes. The author reports on one UK company's perspective of this new market. The company is Southampton-based Bowman Power Systems which has a number of pre-production machines in the 40 to 80 kWe range and is planning the commercial release of its 80 kWe system for the third quarter of 2000. Bowmen has concentrated its development efforts on the electrical components, i.e. the alternator and the power conditioner, relying on a bought in turbine supplied by US-based Elliott Energy Systems. Cogeneration is central to Bowman's marketing approach. The importance of cogeneration is reflected in a distinctive feature of the Bowman design, whereby the heat output can be varied by altering the degree of pre-heating of the delivery air.

The engineering systems approach to small scale combined heat and power

This paper describes some of the factors which must be taken into account when considering the installation of small scale CHP plant. Examples are given of potential and existing installations involving various prime mover types; emphasis is given to the range of applications possible, including linking with heat pumps, chillers, and the use of boost firing.