Many experts believe hydrogen will play an important role in large scale energy storage in the future. There are already some technically usable hydrogen-based solutions, one of which is a photovoltaic-fuel cell system. PEi looks at a pilot system for small scale energy storage which is currently being tested in Finland.
Layout of the seasonal energy storage PV-hydrogen system
A number of companies have long been exploring the use of hydrogen in energy systems.
An interesting technology being commercialized by Fortum Advanced Power Systems, is a PV-hydrogen system for storing electricity. The smaller subsystem, which uses a fuel cell as a standby power source, has been under field test since 1998, while the larger seasonal storage system will be field tested this year.
This type of technology is ideal for remote locations where there is a need for all year round power, especially in places where there is only a small amount of solar radiation in winter.
Like electricity, hydrogen is an energy carrier and not a primary source of energy. However, it has one major advantage in that it can be stored in large quantities for extended periods.
Fortum has been developing a closed loop autonomous energy storage system based on hydrogen technology since 1990.
The basic idea is to use excess PV electricity during summertime for the electrolytical decomposition of water in a pressurized electrolyser. The hydrogen produced is then stored in a pressure vessel, and oxygen is vented to the atmosphere.
During the winter, hydrogen can be used to produce electricity in a fuel cell with the oxygen taken from the air. The water produced by the fuel cell is recirculated back to the electrolyser which closes the loop. Because all the water (“fuel”) cannot be recovered from the fuel cell the system collects the rainwater, which is cleaned with appropriate cleansing agents.
Layout of the fuel cell back-up system for PV applications
The seasonal storage type of PV-hydrogen system is best suited to high latitudes with continuous loads of 50 W-300 W. For larger loads, a hybrid PV-diesel system may be more competitive. The PV-hydrogen system has advantages over other technologies. Batteries are impractical for seasonal energy storage in all but the smallest PV systems. Diesel generator back-up for remote PV-battery systems has the disadvantages of requiring refuelling and maintenance, and there is a lower limit to the size of reliable diesel engines.
For loads smaller than 50 W Fortum is working on using a PV-battery system with just a back-up fuel cell and hydrogen gas storage since the pressurized electrolyser cannot be scaled down to very small systems economically.
This system is simplified from an autonomous seasonal energy storage system by removing the hydrogen production (electrolyser) and water handling system, and retaining the hydrogen-consuming fuel cell.
Insulated fuel cell and electronics enclosure for outdoor use
The gas supply is one or two 200 bar, 50 litre commercial gas bottles, which have to be exchanged every other year.
The design load size is 5-30 W continuous. Typical applications are for powering small weather stations, lighting systems, radio repeaters, traffic signal systems, i.e. where grid supplies are unavailable.
The most critical component is the fuel cell, which must operate in severe cold climate. Fortum is using a Proton Exchange Membrane Fuel Cell (PEMFC), which has a solid electrolyte made from a proton conductive polymer.
The hydrogen gas pressure is 0.5 bar and atmospheric air is supplied to the cathode by a small fan. The hydrogen is supplied directly to the fuel cell without any humidification or recirculation except for some periodic flushing. The current density achieved by the fuel cell using this approach is only moderate but the system is very simple and reliable.
The main technical challenge in the system, is housing the fuel cell for operation in the cold climate. The dry inlet air can increase drying of the membrane and the humid outlet air can form ice.
To study this, extensive laboratory tests were carried out on the fuel cell system down to -25à‚°C. On the basis of the test results, an insulated enclosure was designed for the fuel cell. The enclosure also includes space for the control electronics, so that the entire system is in one box – except for the PV, battery and gas bottles.
The fuel cell could be connected directly to the battery system but using a DC-DC converter gives greater flexibility in the sizing and matching of components. The DC-DC converter can also operate the fuel cell at optimum voltage. Also, problems such as thermal runaway arising from parallel connection of fuel cell connections can be avoided. This can be a problem especially when starting the fuel cell for the first time in autumn when the membrane might be dry. A special control algorithm is used for initial fuel cell start-up.
The prototype system has been under field tests since 1998/1999. While these first field tests were carried out in Lapland, the next field test phase might include systems somewhere else. However, the location has to be in a northern location so that there is a large variation in summer and winter solar radiation levels. Field tests have demonstrated 2-3 month periods of operation without solar radiation.
The main field test was carried out in North Finland (latitude 69 N) and (inset) the field unit in tough climate conditions. The remote site is 30 km from closest road near Utsjoki
All the standard system components (solar panels and batteries) have been tested extensively in the laboratory and in the field. The new hydrogen system specific components (fuel cell, gas handling and control electronics and casing and capsulation of the system) have been tested in the laboratory and in environmental chambers (down to -30à‚°C) under a number of different conditions.
The complete system has also been tested outside since 1996/1997 near Fortum’s technology centre in Porvoo (40 km east of Helsinki). This early test was to discover possible problems and improve the reliability of the design.
The system in Lapland is the first real system involving a customer and the purpose is to test the reliability of the system in a real situation (with the unpredictable aspects).
Also because the weather is harsher in north Finland, Fortum hopes to push the system to its limits. The project is now in the final development and pre-commercial phase. In this phase field tests will be carried out with selected customers during year 1999/2000 in the harsh climate conditions in Finland.