Fuel cells

Karsten Voss, Benoit Sicre and Andreas Buhring 12.6.1 Concept

Fuel cells, like batteries, are electrochemical power sources. Whereas batteries store energy, fuel cells transform energy. A fuel cell steadily supplied with fuel generates electricity. The fuel can be virtually any chemical substance containing hydrogen. When hydrogen alone is not readily available as a fuel, it can be produced from substances such as natural gas, oil or methanol by a process called 'reforming'. Reforming, however, consumes energy, produces waste heat and is generally not emission free. Ideally, hydrogen is produced from water by electrolysis, where the electricity is generated by solar power (solar hydrogen).

Within a fuel cell, hydrogen reacts with oxygen - pure or extracted from the air - and induces an ion flux through the electrolyte (usually an ion-porous membrane). Electrical power is generated while water is produced as a main reaction product (see Figure 12.6.1). This high-exothermic electrochemical reaction can be carried out at various temperature levels, depending on the type of cell (see Table 12.6.1). The temperatures together with the waste heat from the reformer have to be considered for applications with combined heat and power use (CHP). The electrochemical reaction under real conditions induces a voltage ranging from 0.6 V to 0.9 V DC per cell. Therefore, cells are stacked (connected in series) to reach a useful voltage. In a high-performance house, the DC current is converted to AC by an inverter, comparable to those developed for PV applications. With the exception of autonomous houses, the systems are grid connected.

The main advantage of fuel cells is their potentially greater ratio of power to heat generation (power factor) compared to CHP generators driven by combustion engines. The smallest commercial CHP unit currently available features a power output of 1 kW. Thus, such systems are well suited for single family high-performance houses.

Source: K. Voss, Wuppertal University

Figure 12.6.1 The basic electrochemical fuel cell process: Oxidation of hydrogen by means of oxygen

Today, the most promising fuel cell technologies for high-performance single family houses are the PEM-FC (polymer electrolyte membrane fuel cell) and the SO-FC (solid oxide fuel cell). SO-FC systems are still being tested in large-scale field tests (Ballhausen, 2003). The PEM technology primarily profits from the enormous development efforts within the automobile industry. The main weakness for applications in the housing sector is the need for the complex reforming of natural gas to produce pure hydrogen with a minimal content of CO and at a high energy efficiency. The 1000°C needed temperature level of the SO-FC technology results in high thermal stress for cell materials made largely of ceramics. At such high temperatures a more or less continuous operation or hot standby mode is necessary. To improve economy, efforts are under way to decrease the operating temperature down to 800°C. The strength of the SO-FC is the possible internal reforming of natural gas to hydrogen with sufficient purity. This has the added benefit of reducing the overall system complexity.

Table 12.6.1 Characteristic data of the principal fuel cell systems

Polymer electrolyte membrane fuel cell (PEM-FC)

AFC

PAFC

MCFC

Solid oxide fuel cell (SO-FC)

Electrolyte

Ion exchange

Mobilized or

Immobilized

Immobilized

Ceramic

membranes

immobilized

liquid

liquid molten

potassium oxide phosphoric acid

carbonate

Operating temperature (°C)

80

65-220

205

650

600-1000

Charge carrier

H+

OH-

H+

CO3-

O-

External reformer for CH4

Yes

Yes

Yes

No

No

Prime cell components

Carbon based

Carbon based

Graphite based

Stainless based

Ceramic

Catalyst

Platinum

Platinum

Platinum

Nickel

Perovskites

Product water management

Evaporative

Evaporative

Evaporative

Gaseous

Gaseous product

product

Product heat management

Process gas +

Process gas +

Process gas +

Internal

Internal reforming +

independent

electrolyte

independent

reforming +

process gas

cooling medium

calculation

cooling medium

process gas

Source: DOE (2000)

12.6.2 Environmental impact

To fairly judge the overall environmental impact of fuel cells, the reforming process, as well as the material flow from production to recycling, down-cycling or disposal, must be considered. Low emissions of the fuel cell process itself are only one part of the story. In addition, conflicting situations where a fuel cell is used have to be taken into account - for example, the usefulness of 'free' waste heat by fuel cells in a house with a solar thermal DHW system.

A detailed life-cycle assessment of fuel cell systems, currently under development, will provide quantified data on key energy and material flows associated with the manufacturing and operation of fuel cells (Krewitt, 2003). Ecological and economic effects resulting from the introduction of stationary fuel cells into the German energy system are investigated in detail. Long-term scenarios enable the quantification of environmental impacts (for example, changes in greenhouse gas emissions and material flows), and the benefits and burdens on the national economy, as well as the employment effects resulting from a reinforced introduction of decentralized stationary fuel cells in Germany. The study also identifies barriers and appropriate market instruments.

12.6.3 Economics: Status and trends

Fuel cell systems are still very expensive, but the hope is to bring costs down in the not too distant future. Whereas power costs for automobiles are targeted by the US Department of Energy (DOE) at €45/kW (OAAT, 2002), specific costs of approximately €1500 would be acceptable for combined heat and power units in housing (Bornemann, 2002). This favours the assumption that housing is a promising application of this technology. On the other hand, such units must run 40,000 hours, whereas applications for automobiles must only last one tenth of this duration. A disadvantage of the use of fuel cells, compared to heat pumps, is the need for an additional fuel supply. This might be solved by supplying high-performance houses with stored liquid gas, methanol or future special fuel cell 'gasoline' in canisters. It would be worth considering types of fuels appropriate for automobile fuel cells in order to benefit from synergetic effects.

12.6.4 Integration of fuel cells in high-performance houses

Several unique aspects of high-performance housing make applications of fuel cells interesting. While, in high-performance housing, the heating requirement is small, the electrical demand is not necessarily proportionally small. This means that a fuel cell sized to meet the heating demand will not likely generate more electricity than is needed. This shifts the focus from pure electricity generation for the grid to trying to cover the electrical demand of the house itself.

Source: Vetter and Wittwer (2002)

Figure 12.6.2 Basic layout of a fuel cell as the house energy supply system ivith grid connection

Source: Vetter and Wittwer (2002)

Figure 12.6.2 Basic layout of a fuel cell as the house energy supply system ivith grid connection

In order to limit system complexity, the demand for space heating and domestic hot water must be high enough to serve as the sole heat sink for the fuel cell. This low and seasonally relatively constant heat demand makes it possible to not have to have a second heat source for peak power management. A prerequisite for the continuous operation of a low power fuel cell is a high-efficiency, properly dimensioned thermal storage tank.

Source: K. Voss, Wuppertal University

Figure 12.6.3 Energy supply system of the 'self-sufficient solar house'

If the fuel cell is dimensioned for its electrical output, the availability of plentiful 'waste heat' may diminish the motivation for the thermal qualities of the house. This shifts the concern from design driven by conserving heat to total performance design where the concern is minimizing CO2 emissions for all the energy needs of the household.

12.6.5 Early examples

The so-called 'self-sufficient solar house' in Freiburg, Germany, demonstrated how a house could be heated and powered by fuel cell back in 1991. A 0.5 kW PEM-type fuel cell unit was operated with solar hydrogen. Power was supplied to the off-grid house and waste heat was used as winter backup of the solar DHW system. Heat for space heating was produced by direct burning of hydrogen in the ventilation system of the house. Fuel cell operation was triggered by the electricity need of the house, when the battery charge was down. Economics were not considered in this original scientific experience. The whole system achieved an electrical efficiency of 45 per cent (Voss, 1996).

Field tests of fuel cells were initiated by several European utilities in 2002. The pre-market series of solid oxide fuel cell (SO-FC) units of the Swiss manufacturer Sulzer Hexis were used in one of these test series. Identical units were installed and monitored in buildings of various types and energy demand levels. Considering the need for continuous operation and the requirement that an identical unit serve various house types, it was decided to integrate a gas burner of typical household sizes as part of the unit. The gas burner meets the peak heat demand, whereas the fuel cell covers the base heating load.

Table 12.6.2 Key system data of Sulzer Hexis HXS 1000 Premiere

Power capacity, fuel cell

1 kW

Heat capacity, fuel cell

3 kW

Backup burner capacity

12/16/22 kW

Electric efficiency

25-30 per cent

Total efficiency

Approximately 85 per cent

Boiler volume

200

Fuel

Natural gas

Dimensions

1080 mm x 720 mm x 1800 mm

Weight

350 kg

Source: www.hexis.com

Source: www.hexis.com

Note: Where natural gas is used, additional energy flows and losses would have to be considered for the reforming process.

Source: K. Voss, Wuppertal University

Figure 12.6.4 Measured energy flow diagram of the polymer electrolyte membrane (PEM) fuel cell operated with hydrogen

12.6.6 Conclusions

Natural gas-fed fuel cell systems have achieved an end prototype phase adequate for large-scale field testing. However, substantial technical improvements are still needed before market entry will be possible.

Durability, reliability and the lifetime of the components need to be improved, especially for the high temperature systems. Currently, cell degradation (primarily corrosion or malfunction of components) limits the practical operating life fuel cells. Moreover, the energy consumption of the parasitic components (typically, fans and pumps) must be reduced. When these challenges are overcome, fuel cells will be an attractive means of producing electricity and heat in the context of housing (Vetter and Sicre, 2003).

Although natural gas-supplied fuel cells have a lower environmental impact than some competing technologies, decisive emission reductions are unlikely as long as fuel cells must be supplied by a fossil fuel. A transition, making use of biogas, would improve the environmental impact of fuel cells substantially. The construction of nation-wide grids supplying solar hydrogen seems totally unrealistic at current costs; but unrealistic today may look different in 25 years.

The potential of fuel cell applications in high-performance housing is promising since the power-to-heat ratio of such buildings better matches that of fuel cell CHP systems. Simulation results show clearly that no additional gas heater is required since the heating peaks can be easily covered by an electrical heater without jeopardizing the primary energy balance of the house.

Source: Hexis AG, Winterthur, www.hexis.com

Figure 12.6.5 The Sulzer Hexis system

References

Ausschuss (2001) Brennstoffzellen-Technologie, Bericht des Ausschusses für Bildung, Forschung und

Technikfolgenabschätzung, Drucksache 15/5054, Berlin, Germany Ballhausen, A. (2003) Energiedienstleistuung mit Brennstoffzellen - Contracting-Lösung für Privatkunden, Proceedings of the Conference Brennstoffzellen-Heizgeräte zur Energieversorgung im Haushalt, Haus der Technik, Essen, Germany Bornemann, H. J. (2002) Hybrid Power: A European Perspective, Second DOE/UN Workshop and International Conference on Hybrid Power Systems, www.netl.doe.gov/publications/proceed-ings/02/Hybrid/Hybrid2Bornemann.pdf Britz, P (2002) Das Viessmann Projekt Brennstoffzellen-Heizgerät zur Hausenergieversorgung;

Proceeding of the Fuel Cell Conference Haus der Wirtschaft, Stuttgart, Germany DOE (US Department of Energy) (2000) Fuel Cell Handbook, fifth edition, Science Applications International Corporation, US Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Morgantown, West Virginia, US Krewitt, W. et al (2003) Policy Context, Environmental Impacts and Market Potential of the Application of Decentralised, Stationary Fuel Cells, Proceedings of the Hannover Messe, Hannover, Germany OAAT (Office of Advanced Automotive Technologies) (2002) Office of Advanced Automotive Technologies Cost Model Identifies Market Barriers to PEM Fuel-Cell Use in Automobiles; DOE, www.cartech.doe.gov/research/fuelcells/cost-model.html Vetter, M. and Wittwer, C. (2002) Model-based Development of Control Strategies for Domestic Fuel Cell Cogeneration Plants: Proceedings of the French-German Fuel Cell Conference 2002, ForbachSaarbrücken

Vetter, M. and Sicre, B. (2003) Sind Mini-KWK-Anlagen für das Passivhaus geeignet? Anforderungen und

Potenziale, Beitrag zur Passivhaus-Tagung 2003, Tagungsband, Hamburg, Germany Voss, K. (1996) Experimentelle und Theoretische Analyse des Thermischen Gebäudeverhaltens für das Energieautarke Solarhaus Freiburg, Thesis, Ecole Polytechnique Federale de Lausanne, Switzerland

Website

Hexis Ltd: www.hexis.com

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