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Reduction of residential carbon dioxide emissions through the use of small cogeneration fuel cell systems

Background

Combined heat and power systems (CHP) enable users to operate with increased overall efficiency thus reducing consumption of fossil fuels and green house gas emissions. This study investigates the potential of fuel cell based CHP systems in domestic and small commercial applications to reduce green house gas emissions. Fuel cells in this application have advantages of very low emissions and potentially higher power to heat ratios than other CHP systems.

Study approach

The study was undertaken by the Systems Analysis and Technology Evaluation (STE) Department of the Institute of Energy Research (IEF) at the ForschungZentrum Jülich, Germany.. The recent trends in energy consumption and CO2 emissions for the EU 251 in the industry, residential and commercial sectors were first analysed to get a good understanding of the baseline situation and the typical energy consumption trends in developed countries. The main part of the study consisted of four steps. Firstly, the characteristics of all types of CHP systems on or moving to the worldwide market including those based on fuel cell technology were researched and documented. In the second part the characteristics of heat and power consumption in houses were analysed for three climatic zones; cold, temperature and warm. This analysis was based on both a top down and bottom up approach. In the latter, estimates of heat consumption were made based on the size and insulation standards of typical housing units and then compared with the overall consumption of energy for heating in the sector as a validity check. The third step was to analyse the moment to moment consumption patterns of heat and electricity in order to estimate how fuel cell and other CHP systems would perform in practice. This information was then used to calculate the emission savings which could be made when fuel cell and other CHP systems were applied in place of the conventional system of supplying centrally generated electric power for the electrical loads and gas for heating and hot water. In the fourth and final step assessments were made of how great the uptake of fuel cell CHP systems to 2050 would be and hence what overall contribution they could make to greenhouse gas emission reduction. The potential of the fuel cell systems was then compared with other CHP alternatives. The work was largely based on European energy and building statistics but is considered generally applicable to OECD countries. The full analysis was not extended to the commercial/industrial sector due to lack of readily available information on the diverse applications in this sector.

Results and Discussion

Trends in energy consumption and emissions

Figures for energy consumption in the EU 25 as published by EUROSTAT 20072 were reviewed in order to understand the basic trends in the industrial, residential and commercial sectors. In the period 1990 -2004 overall energy consumption in the 25 countries which now make up the EU rose by 7.6% but the residential sector climbed by 15% and the commercial sector by 29%. There was also a considerable switch to gas in the residential sector and significant increases in electricity consumption which was up by 31%. Likewise, the commercial sector in the EU shows a similar pattern of significant increase with the clean fuels electricity and gas gaining significant market share. The ratio between electricity and power consumption is a key parameter when considering the potential of CHP systems in the EU. Also of importance in assessing the contribution to emission reductions is the baseline emission characteristics with which the technology will compete. For the EU25 statistics the average figures for electricity and thermal energy production are calculated using emission factors for the average EU fuel and energy mixes in 2004 and were electrical 0.35kgCO2/kWhel and thermal 0.37 kgCO2/kWhth Charts showing all of the trends are to be found in the main report.

The advantages of CHP

Combined heat and power (CHP) means the simultaneous generation of thermal and electrical power in one system. In comparison to separate generation of heat in a domestic heating system and drawing of electricity from the public electricity grid, CHP-systems have the potential to save primary energy. The main reason for this energy saving potential is the use of the waste heat which is normally rejected by thermal energy conversion systems. For small decentralized CHP-systems (often termed micro-CHP), avoiding network losses is an additional positive aspect. Figure 1 below illustrates the potential savings in primary energy when the requirements are 1 unit of electrical power to 2 of thermal energy. The conventional system in this case uses 53% more primary energy based on typical central power generation and transmission with 35% efficiency. Even if the electricity system efficiency is pushed up to 50% the conventional system would use 27% more primary energy.

Figure 1 Comparison of Conventional and CHP system energy balance

Survey of available micro CHP technologies

Micro CHP systems have been developed based on electricity generation using at least 5 methods listed below. Examples of the characteristics of systems based on all of these apart from the organic Rankine cycle were obtained and selected data from the range of available systems was subsequently used in the estimation of abatement potential. The number of specific designs for which data was collected is shown in brackets.

  • Fuel cells (20)
    • Alkaline (1)
    • Proton exchange membrane (PEM) (7)
    • Solid oxide (SOFC) (5)
    • Molten carbonate (4)
    • Phosphoric acid (3)
  • Internal combustion engines (3)
  • Stirling engines (4)
  • Micro gas turbines (5)
  • Organic Rankine Cycle (No commercial examples found)

The models investigated were as follows: (Note: more extensive details of their characteristics and performance and pictures of the units are to be found in the main report)

Intensys produce the PULSAR-6 alkaline fuel cell with an electrical capacity of 6Kwe

The 7 PEM fuel cells reviewed range in capacity from 1 to 5Kwe and are made by the following manufacturers:
Vaillant-PlugPower 5Kwe
Inhouse 4000 4Kwe
Viessmann - HEVA II 2Kwe
Baxi Innotech -Beta 1.5 1Kwe
Matsushita Electric Industrial 1Kwe
Toyota-Aisin 1Kwe
The 5 SOFC systems reviewed are all small capacity:
Ceramic Fuel Cell Ltd NetGenPlus 1Kwe
Hexis -Galileo 1000N 1Kwe
Acumetrics-AHEAD 1Kwe
MTS/Elco/Acumetrics 1Kwe
Kyocera/Osaka Gas 0.7Kwe
These PEM and SOFC fuel cell based systems listed above are the main contenders for the domestic CHP market. The main competitor at this power output level is the Sterling engine based CHP system of which 3 examples investigated.
Whispergen Mk5 1Kwe Gas fired
Solo Stirling 161 2 – 9.5Kwe Gas fired
Sunmachine 1.3-3Kwe Wood pellets
Reciprocating internal combustion engine systems are also available with outputs suitable for the domestic market:-
Senertec (Baxi) –DachsSEplus 5 - 5.5 Kwe
PowerPlus Technologies (Vaillant) Ecopower 1.3 - 4.7 Kwe
Honda -GE160V 1 Kwe

Molten carbonate (MCFC) and phosphoric acid fuel cells (PAFC) are also available but the commercially available units are larger in size and are more suited to the small commercial CHP market. Finally a number of systems based on micro gas turbines are available in the larger capacity ranges. These were not used in the detailed analysis of abatement potential and their characteristics are listed in the main report.

Heat and power demand in the domestic sector

Heat demand

The heat and power demand was analysed for two types of dwelling, single family houses and multi family housing units in which a heating system is shared, for example in a block of flats or a terrace of houses. Using a “bottom up” approach the heat requirements over the year were estimated for a “standard” single family house using data on the areas of walls roof and windows and insulation coefficients taken from the applicable standards. These calculations were made for three different climatic zones based on the “heating degree days” profiles for three representative countries namely Finland (cold), Germany (moderate) and Greece (warm). This resulted in typical heating load curves as shown in Figure 2.

Figure 2. Heat demand single family house from bottom up analysis for 3 climatic regions

The validity of this approach was checked by doing a “top down” calculation which uses total thermal energy demand for the countries concerned combined with information on the number of houses and the split between single and multifamily. Comparison of the results from this approach showed agreement to within roughly 13-16%, the bottom up estimates being consistently higher. Over time the heat demand in houses is expected to change. A major effect will be the rapidly improving standards of insulation in new houses. Figure 3 illustrates the changes which have occurred historically and the very low heat losses per unit area of a house which are expected to be reachable in the future. There is a balancing trend however towards having an increasingly large amount of the living space per house heated. There is evidence that floor areas of newer houses are slowly declining but the effect of increased area heated applies to the whole of the housing stock. The net result is a slight increase of expected heat demand per house over time.

Figure 3 Trends in insulation standards

Electrical demand

The overall performance of a CHP system is very dependent on the balance between the electrical and thermal power demands. For domestic housing the electrical load varies considerably as shown in the example in Figure 4 below which is based on measurements every 15 minutes. Notice that for the aggregate of 83 units in a multi-family house the load is more evenly spread.Because electricity is difficult and expensive to store any calculations on CHP in the domestic application should ideally be based on much more frequent data on the electrical consumption. The researchers found a severe lack of such short time scale data.An example for one single family house in which power was measured every 10 seconds was found and data from this source was used to simulate electrical consumption using a random simulation model. Such data would be important for a power led CHP system. However for the calculations of CO2 abatement potential a heat lead system is adopted and a small hot water storage is included which has the effect of smoothing out the heat production from the CHP system. Surplus electricity is exported to the grid. Based on typical heat and electrical load profiles the operating hours of the chosen systems were calculated including details of the requirements to stop and start and to use a peak burner for any shortfall in thermal output.

Figure 4 Typical daily household electrical load – 15 minute measurement interval

Emission reductions and costs per unit

General parameters

Based on the heat demand for typical single and multi-house units the annual emission reduction and emission reduction costs for these units are calculated. The results of these calculations are dependent on a number of parameters which are expected to change over time. For example the emissions of CO2 for grid produced electricity, the cost of its production, the cost of micro CHP systems are all expected to change in the period up to 2050. Figure 5 which is based on data from the IEA for the OECD region shows the changes expected in these parameters compared to a base year 2010. The parameters are incorporated into the overall model to calculate emission reductions and costs.

Figure 5 How key parameters are expected to vary with time

Fuel cell performance

The average performance of SOFC and PEM fuel cells currently available is shown in the left hand side of the table below. It is expected that these values will be significantly improved by the time the devices enter the mass market and in the columns on the right show the values assumed for the calculation future emission abatement potential. Note in particular the significantly higher electrical efficiency of the SOFC and somewhat improved overall efficiency of the PEM fuel cells.

Current average efficiency [%] Efficiency for calculations
Type electric thermal overall electric thermal overall
SOFC 35.8 46.8 82.5 45 30 80
PEMFC 29.7 44.1 73.9 30 50 80

The calculation of relative performance per unit for different types of fuel cell CHP as compared to competing systems is shown in the following charts (Figures 6 & 7). The results are shown for periods 10 years apart from 2010 to 2050. For the multi family house the comparison is made between a CHP based on an internal combustion engine and a low temperature PEM. For the single family house the comparison is between a Sterling engine type CHP a SOFC and a PEFC. In all cases but one there is a slight increase with time as a result of the scenario assumptions. In the case of the PEFC in a single family house there is a slight decrease in savings in later decades because improvements in the reference grid electricity emission factor start to outweigh the benefits from the relatively small amount of CHP electricity which this type of unit produces.

Figure 6 Abatement potential of various CHP systems per housing unit

The corresponding chart (Figure 6) for the avoidance costs shows that economies of scale play a significant role so that the abatement costs for multi-family houses are much lower or negative than for single family houses. Also the relatively high cost of very small fuel cell systems makes for rather high avoidance costs. All of the above figures are based on the assumption that starting and stopping the units does not affect their assumed overall efficiency. Because of the high operating temperature some restrictions were brought in for the SOFC system so that in the simulation the unit could not be restarted for some hours after a shutdown. It is evident that unless there can be major cost breakthroughs the fuel cell systems are at a serious cost disadvantage compared to competing CHP systems based on Sterling engines.

Figure 7 Cost per ton of CO2 abatement using various CHP systems

Market penetration

In order to build up full scenarios for which abatement costs and amounts can be calculated it is necessary to make estimates of the number and capacity of fuel cell CHP systems which will be sold into the market. The study uses results for a market survey of heating appliances for the world market made by Bosch Thermotechnik GmbH in 2006.3 This makes projections as to the total market and also the make up of that market. After examining information from manufacturers on progress with the development of fuel cell micro CHP systems it was considered reasonable to expect significant market entry in the OECD by about 2014. Considering the breakdown of the market and the competitiveness of other systems two cases for market penetration were selected, a low case of just 5% and a high case of 20%. A standard logistic function was used to assess the trajectory of the penetration. This was calibrated so that full market share was reached by 2030 being achieved by an initial exponential rise followed by a levelling off which is typical of such markets. Post 2030 the technology was assumed to maintain a constant share of the total market which continues to grow steadily through to 2050. The trajectories are illustrated in Figure 8.

Figure 8 Market penetration profiles

Overall abatement potential

Having established all these conditions the total potential for the OECD market was estimated. This is considered to be the realistic extent of the market with capability to adequately support deployment of this technology for domestic consumers. Under these assumptions the CO2 reduction from the deployment of Fuel Cell Heating Appliances in the residential sector can reach between 14 to over 50 million tonnes of CO2 per year by 2050. The trend in this potential is illustrated in Figure 9. To put this amount in perspective it should be noted that this corresponds to a reduction of emissions of between 1% and 4 % of the emissions in the residential sector of the OECD. Whilst significant this is a relatively modest contribution and is rather sensitive to the key parameters on which the scenario calculations are based. For consideration of different climatic zones in the OECD an arithmetic average system for the warm, moderate and cold zone was used. The CO2 avoidance of the systems to the reference is calculated for the OECD mix of power plants with average emissions for grid electricity derived from IEA data4.

Figure 9 Potential CO2 emission reduction from domestic fuel cell CHP systems in the OECD

Expert Reviewers Comments

The expert reviewers found the report to be detailed and thorough. Some were concerned about the rather small potential emission reductions which were calculated and felt that the assumption of a purely heat lead system was too restrictive and that better results might result if better use was made of the electrical capacity. On the other hand another reviewer commented that the reduction figures for Sterling engine CHP found from practical tests were much lower than the predictions in this report. The reason was considered to be that the overall efficiency of such micro CHP systems is considerably reduced in practice by frequent stops and starts. Thus results based on performance information for steady state operation could be seriously misleading. The same reviewer also commented that the need to have a hot water storage tank to smooth out the thermal heat production was a serious disadvantage in the domestic market place where space in domestic properties is at a great premium. This could render the assumptions for future market penetration rather optimistic.

It was commented that the insulation standards chosen for analysis of heating requirements in the moderate and cold regions were rather stringent compared to what might be typical in the OECD. Also, that the heat insulation calculations had not taken account of heat losses due to ventilation which can be significant. A more sophisticated calculation of heat loads for typical housing units would be possible but ultimately when totalised this has to agree with measured total actual residential consumption figures. It is on these latter figures on which the projected savings are based.

Reviewers also cautioned that consumer behaviour might work to reduce the calculated emission cuts since the availability of cheaper heat after a CHP system was installed would encourage its more profligate use.

There was general disappointment that fuel cell CHP systems seemed to offer so little potential for reduction of emissions but also acceptance that there were good reasons for this conclusion. It was also noted that the natural gas system might need to be significantly extended to supply the energy needed for distributed generation of electricity and given the limited emission reduction potential a better option would be to go for central decarbonisation with distribution of either hydrogen or more electricity. It was commented that fuel cells might come into their own if there was a hydrogen distribution system. However consideration of such a change in infrastructure was beyond the scope of this report.

Reviewers also commented that the report had not been able to make predictions for the reduction potential in the commercial facility market. This shortcoming is recognised but the sector is considered to be much more diverse that a convincing estimate of potential emission reductions would require a much more extensive survey of the opportunities, which could not be undertaken with the resources available for this study.

Conclusions

The main conclusion is that fuel cell CHP systems can only be expected to make a small contribution to emissions reductions in the domestic housing energy market in the future. Their potential contribution is very sensitive to the carbon intensity of the electrical power supply and if this decarbonises substantially would eliminate any of the advantages for domestic CHP systems consuming natural gas.. Fuel cell CHP systems still suffer from high projected costs compared to competing CHP systems and unless this disadvantage can be effectively addressed they will struggle in the cost sensitive domestic market place.

The report has not looked at the potential for fuel cell CHP were centrally produced hydrogen to become widely available. This would tend to improve their performance relative to competing systems. However analysis of this type of system would require development of rather speculative scenarios involving a shift to a hydrogen economy. There would be potential competition from a clean electricity based economy perhaps involving more extensive use of heat pumps which again would be a rather speculative scenario on which to base calculations.

Recommendations

It is recommended that no further work is done on analysis of the potential of fuel cell systems to reduce emissions for the time being.

1 EU 25 are the 15 original countries in the European Union plus the 10 countries who recently accessed.

2 EUROSTAT is the Organisation which produces and publishes official statistics for the European Union

3 BBT (2006) Marktreport 2006 - Energie effizienter nutzen. BBT Thermotechnik GmbH, Bosch Gruppe. www.bosch-thermotechnik.de/sixcms/detail.php?id=2326456, 2006

4 IEA Energy Technology perspectives 2006, Scenarios & Strategies to 2050. OECD.
www.iea.org.
IEA World Energy Outlook 2006. OECD/IEA.

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Technology