The recent announcement of the development of a mini-grid powered by a fuel cell-battery combination points to a new area of possible applications for fuel cells, which have to date primarily been used in standby applications.
“Sir William Grove, a Welsh judge and gentleman scientist, developed the fuel cell concept in 1839, but the invention never took off. This was in part due to the rapidly advancing internal combustion engine, which promised better results. It was not until the 1960s that the fuel cell was put to practical use during the Gemini space program” [1].
Hydrogen fuel cells have developed since then into a product used widely in many different applications. For many years the main application was in small systems, such as primary and back-up power sources for remote telecommunications, the product has now found application in many other areas such as hydrogen batteries, in combination with electrolysers, in transport for hydrogen powered vehicles and in primary power generation sources, where units up to 1 MW capacity are commercially available. Several such units are being used for peak shaving (demand management) applications worldwide [2].
Self contained modules are available in sizes ranging from 1 kW to over 100 kW, and modules can be combined to provide an output of any size required. There is a growing interest in the production of hydrogen from renewable sources, and the hydrogen fuel cell could an important part in the future energy mix.
The hydrogen fuel cell
Several varieties of fuel cell have been developed, but the most common in use today is the proton exchange membrane (PEM) fuel cell. Solid oxide fuel cells which operate at high temperatures, are making an impact on the market, but are still under development.
Operation
The construction of a typical PEM fuel cell is shown in Fig. 1.
Fig. 1: Proton exchange membrane fuel cell.
The cell uses a solid polymer membrane as the electrolyte. The anode and cathodes are both made of catalytic materials, generally platinum based. Hydrogen gas is fed into the anode and atmospheric oxygen into the cathode. Not all the hydrogen fed into the cell is converted on a single pass and unused hydrogen is recycled through the cell. The reactions in the cell take place at room temperature, although high temperature PEM fuel cells are being developed which give improved performance.
The reactions are as follows :
Anode reaction: 2H2 → 4H+ + 4e- (1)
Cathode reaction: O2 + 4H+ + 4e- → 2H2O (2)
Overall cell reaction: 2H2 + O2 → 2H2O (3)
Operational lifetime
The lifetime of a fuel cell will depend on the application and is estimated to range from 2000 h in standby operation to over 50 000 h in continuous operation [3].
Start-up time
PEM fuel cells are not instantaneous in operation but require a short start-up time to reach full output. Start up times depend on the size of the module.
Sources of hydrogen
Methanol reformation
Methanol is often used as a liquid fuel for smaller portable hydrogen fuel cell systems as it has a number of advantages over bottled hydrogen, mainly that it is liquid at normal temperature and pressure and can be transported without special precautions. Hydrogen is derived from methanol by a process of reformation, the methanol reformer often being incorporated into the fuel cell module. The process of methanol reformation is as follows:
Methanol fuel mixed with water is vapourised at high temperature and passed over a catalyst to form hydrogen and carbon dioxide
CH3OH + H2O → 3H2 + CO2 (4)
The process is incomplete and the excess methanol is recycled and used to provide heat for the reformation process. Research is ongoing into the optimal use of catalysts for the process. Methanol is normally produced from natural gas or syngas but can also be produced from renewable resources such as biomass, agricultural and timber waste, solid municipal waste, landfill gas, industrial waste and a number of other feedstocks [4]. This offers great potential for the fuel cell as a source of renewable energy generation.
Hydrocarbons
The majority of hydrogen used in industry is produced by the steam reformation of methane in natural gas or syngas. This process produces CO2 and is therefore cannot be seen to be green or renewable. The cost of hydrogen from these sources is also dependant on the cost of the primary hydrocarbon.
Biogas
Biogas from many renewable sources can be reformed by various process to produce hydrogen. Currently landfill sites and municipal waste water treatment plants are using gas engines to generate electricity, but fuel cells may offer a more efficient means of generation as well as producing heat for use in the plant.
Chemical industry waste-gas
Hydrogen is produced as a waste product by several processes in the chemical industry. The main source is the production of chlorine and sodium hydroxide from brine. Waste hydrogen from these processes is either vented to the atmosphere or used for the production of heat required for other processes. The process produces 340 m3 of hydrogen for every 1000 kg of chlorine produced [5]. This is sufficient to generate 425 kWh of fuel cell electricity. The Canadian hydrogen and fuel cell association (CHFCA) estimates that Canada vents over 50-million kg of hydrogen annually, sufficient to provide approximately 90 MW of base-load generation, and that there are over 1000 sources of by-product or waste hydrogen worldwide. A factory in Canada produces 600 to 1000 kg/h of waste hydrogen, sufficient to generate 10 to 14 MWh/h of electricity if used in a fuel cell [6].
This may create the impression that a potential exists to generate bulk utility electricity from waste hydrogen, but this is fraught with uncertainites, the main one being that should the primary process be shut down, cease operation or decrease output for any reason, the source of fuel disappears. This applies to the use of any industrial waste. The use of waste hydrogen would therefore judiciously be confined to internal power generation for own use, perhaps with the sale of “surplus” electricity, as and when available, to the utility.
Electrolysis
Hydrogen can produced by electrolysis of water, and at present about 5% of the world’s hydrogen is produced by this method. The drawback to applying this method on a large scale is the cost of electricity. The cost of this resource is escalating and with electricity shortages in many countries, particularly in Africa, the use of grid electricity to generate hydrogen for fuel cells on a large scale can hardly be justified. There is however an alternative, in the form of renewable energy resources such as wind and solar power. There is a pervasive belief circulating in the industry that “surplus electricity” from these sources can be obtained at a low price and used for hydrogen generation.
This ignores the fact that the surplus electricity is the result of mandatory off-take requirements placed on the network operator or purchaser and not on any market conditions. This requirement has resulted in the spot price of electricity going very low and sometimes even negative in some countries, but the producer of this surplus electricity is not being paid the spot price but a contracted amount. Surplus renewable energy may be cheap to purchase but is not cheap to produce. This situation may change in future resulting in the disappearance of cheap surplus electricity which may have a significant effect on the viability of electrolysis. This also ignores the variable nature of both wind and solar which would make the operation of most electrolysers difficult.
Not withstanding this, a dedicated solar or wind power plant may at some time be able to produce electricity at a lower cost than grid electricity, which may lead to the viable production of renewable hydrogen, and attain the goal of renewable hydrogen.
Nuclear thermochemical decomposition of water
Hydrogen could be produced by high temperate steam electrolysis (HTSE) using heat from a nuclear reactor and solid oxide electrolyser cells. This has been proposed by numerous supports of nuclear power.
Hydrogen storage and transport
The big challenge facing the use of hydrogen as a fuel is storage and transport. At room temperature hydrogen is very light gas, and in gaseous form very explosive (a 4% mixture of hydrogen in air is considered to be explosive). Hydrogen is normally supplied to industry in cylinders at a pressure of 200 bar. Hydrogen powered vehicles store compressed hydrogen at a pressure of 300 to 700 bar. Storage tanks for hydrogen are thus much heavier than tanks for other gases.
Various methods have been investigated for storing hydrogen in a form other than compressed gas. Metal hydrides and other various compounds have successfully been put to this use. A recent development of diesel-like hydrogen fuel, known as a liquid organic hydrogen carrier is receiving a lot of attention, and could provide a solution to the problems of hydrogen storage.
The hydrogen battery – bulk storage of electricity
The primary interest in the hydrogen power cycle for the power generation industry must be the use of hydrogen for bulk energy storage. Storage is becoming an increasingly important factor in renewable energy generation, and is regarded by many as the next hurdle to overcome in the growth of renewable energy in the power generation field. With wind and solar farms having outputs of hundreds of MW, storage requirements to counter the variability of the resources are in the region of tens if not hundreds of MWh. The hydrogen battery offers the capacity required to store the amount of energy needed, but there are concerns about performance.
There are a number of small capacity hydrogen batteries on the market, but as yet none of the capacity required for a large RE installation.
Round trip efficiency – end to end efficiency (RTE)
An important performance parameter of any storage device is the “round trip efficiency”. This looks at the overall efficiency of the process where hydrogen is generated by electrolysis using electricity. Other methods of generating hydrogen will not be considered, and have their own efficiencies. For instance, where bio-methanol is used, one could consider the end to end process of capturing energy from sunlight to the final conversion to electricity.
Round trip efficiency compares the amount of electricity recovered from the total process to the original amount of electricity consumed by the total process. The calculation that follows ignores energy losses due to compression and other processing of stored gas and considers the efficiency as if there was a direct connection between the electrolyser and the fuel cell.
Fig. 2: Round trip efficiency.
To calculate the overall efficiency we need to look at the performance of both the fuel cell and the electrolyser. Fuel cell manufacturers are fond of quoting the efficiency of the cells in converting the available energy in hydrogen into electrical energy, and figures in the mid 50% range are often quoted. This is of little value to us though, and the figure which we shall use is the hydrogen consumption per unit of electrical energy produced. There is wealth of information available from manufactures, with most systems having hydrogen consumption figures of the order of 0,8 Nm3/kWh for small to medium systems (5 to 100 kW) and 65 kg/MWh for larger systems up to 1 MW and above.
We then need to combine this with the energy used per unit of hydrogen produced. A survey of the electrolyser market shows values of the order of 4,5 to 5,5 kWh/Nm3 or 50 to 60 kWh/kg of hydrogen produced .
There are two methods of approaching this:
The first is to start with the output and calculate the hydrogen required per unit output.
1 MWh of output requires Mkg or Nm3
The next step is to calculate the electricity required to produce that amount of hydrogen. If Eh is the energy required to produce 1 unit of hydrogen then the energy required to produce M units will be:
Ein = M*Eh MWh (5)
And the efficiency will be :
Output/input = 1/ Ein.= 1/M*Eh (6)
The second method yields the same results but starts with the input energy, calculates the amount of hydrogen that would be produced by a unit (say 1 MWh) of electricity, and then calculates the amount of electricity generated by that amount of hydrogen.
Electrolyser conversion = H (kg or Nm3)/MWh input
Fuel cell consumption = M ( kg or Nm3)/ MWh
And the power output per unit fuel consumed would be: 1/MMWh/(kg or Nm3)
The output energy would thus be: H/MMWh/MWh, which would also give the efficiency.
System size
Electrolyser consumption
Fuel cell consumption
Round trip efficiency (%)
Small to medium
4,5 to 5,5 kWh/Nm3
0,8 Nm3/kWh
23 to 27
Large
50 to 60 kWh/kg
65 kg/MWh
26 to 30
Table 1: Round trip efficiency for hydrogen battery system.
Table 1 shows the results using performance figures for several combinations of electrolyser and fuel cell available on the market. The figures are all in the mid to upper twenties, which means that for every unit of electricity put into the system, only roughly a quarter is retrieved, or for every unit of electrical energy extracted from the system, four units are input.
This places serious questions on the use of electrolyser produced hydrogen for power generation and definitely for bulk storage of surplus electricity. These figures do not compare well with chemical storage batteries with around trip efficiency in the region of 80%. These figures apply to cases where hydrogen generation and usage are separate. Integrated units which combine generation, storage and re-use in a single module may achieve higher round trip efficiency.
Viability of hydrogen power
The growth of hydrogen power in all applications is going to depend on the availability of cheap hydrogen. Outside of mandatory carbon free applications, High reliability power sources, and remote site or mini-grid applications, the cost of hydrogen is going to limit the large scale application of hydrogen based power systems, including bulk energy storage.
High temperature reversible solid oxide fuel cells (RSOFC) would appear to offer much higher round trip efficiencies than PEMFCs but actual figures are not easy to obtain as most products seem to be in development stages. There is considerable research taking place on this technology and RSOFC seem to offer a great promise for the future for CHP or even cogeneration applications.
Nuclear power may be another source of cheap hydrogen, and the future generation of high temperature reactors could be custom built for hydrogen production.
References
[1] Battery University: “Fuel Cell technology”, http://batteryuniversity.com/learn/article/fuel_cell_technology
[2] Ballard power systems: “Distributed generation”, www.ballard.com/fuel-cell-applications/distributed-generation.aspx
[3] P Beckhaus: “Influence of operation strategies on the life time of PEM fuel cells”, www.zbt-duisburg.de/fileadmin/user_upload/01-aktuell/05-publikationen/05-vortraege/2013/fcell-vortrag-beckhaus-v7_ALNpbsx.pdf
[4] Methanol institute: “Renewable methanol”, www.methanol.org/Environment/Renewable-Methanol.aspx
[5] University of York: “The essential chemical industry online: Chlorine”, www.essentialchemicalindustry.org/chemicals/chlorine.html
[6] Canadian hydrogen and fuel cell association (CHFCA): “Case study: capturing waste hydrogen”, www.chfca.ca/say-h2i/distributed-power/case-study—capturing–waste–hydrogen
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