The hybrid power supply needs a central and reliable control to run the system closely to the predefined rules. It also calculates system states like the battery’s SOC and State-of-Health, the current End-of-Charge-Voltage etc.. All system states, setup values and system parameters reside in a small scale database, with capabilities of automated backup to nonvolatile memories.

Component control has been implemented by the mean of state machines. A control kernel frequently checks if certain conditions (e. g. limits exceeded, inputs set, times elapsed etc.) are fulfilled to bring a component from one state to another. On entering a new state a list of tasks is executed (e. g. switching outputs, set values etc.). This way changes to the components can easily applied without programming and future reuse of code is possible.

The EMS has interfaces of different kinds (digital, analogue, serial) to all components it is attached to, shows important information on a display and provides a serial-line interface for setup, maintenance and continuous monitoring or logging of data. Via this connection it is possible to remotely control the power supply system from an operation and maintenance centre (e. g. by the telecom service provider).

The features described are all implemented within a low cost microcontroller platform, using a robust embedded operating system, robust to power losses.

Operating experience

The system has been set into operation during spring 2003. Overall system behaviour has been reported by Ciemat, analysing the energy flow within the system [1]. Operation data of the period from May to September 2003 will be presented in the following section. In the summer period, the fuel cell stack has been running only for frequent self-tests. Thus, we will concentrate on the data of the hydrogen production by electrolysis.

The electrolyser is started by the EMS system if the calculated state-of-charge of the battery reaches a certain value. This value is a linear function of daytime. Operation is possible from 9 am till 4 pm and only during summer months (March — October). In order to prevent the electrolyser to be damaged by a high operating voltage, the battery voltage is reduced with the help of the charge controller to 56.4 V which equals a voltage of 1.88 V per cell. Additionally, premature aging of the battery is avoided by the following rule: Electrolyser operation is only possible if the relative PV power is above 65% of the nominal PV generator power. These limitations and their value are a result of the opimisation of the life-time cost (TALCO).

Figure 3 shows the typical operation behaviour of the of the electrolyser in the test field for one day. The electrolyser starts at around noon after the battery has been loaded during the morning from the PV panels. The voltage of the electrolyser corresponds to the voltage of the battery as the electrolyser is directly coupled to the battery. The constant rise in electrolyser current shows that the electroctrolyser has not yet reached its stationary operation point. At 4 pm the electrolyser is shut down by the EMS following the rules as mentioned above.

The electrolyser did show less —o— I Hectrolyser —■— SOC ♦ U Battery —O— U Electrolyser

Figure 3: Typical day of operation with H2 production

current flow as expected. In laboratory tests, the current at operating conditions (70°C,

30 bar) was 20 Ampere, whereas the current reached only a maximum of 11 Ampere in the

test field. An analysis of the data ^

shows for this effect to be mainly due to the temperature behaviour of the electrolyser stack.

The characteristics of electrolyser cells for different temperatures are shown in Figure 4. As can be seen, the current is very sensitive to the operation temperature.

Compared to an operation temperature of 70°C, only 70% of the current density can be reached at 40°C. As the current is

directly coupled to the hydrogen production rate (see (1)), this leads to a reduction in hydrogen production of the same amount.

There is no heating implemented in the electrolyser system. To reach the operating temperature the waste heat of the process of electrolysis is used. The higher the efficiency at the operating point, the lower is the energy that is converted into heat. The efficiency of an electrolyser cell is defined by U0XN (2)

hEL = 0

U

VH = 0.418^ xI (1)

■ 25°C • 40°C 70°C Figure 4: UI characteristic of electrolyser cells

where U0 is the open cell voltage (1.23 V), N the number of cells in the stack, and U the operating voltage. Thus, the operating voltage has a direct influence on the temperature behaviour of the stack.

As the electrolyser is directly coupled to the battery, low battery voltages lead to a slow temperature increase of the electrolyser stack. Figure 5 shows the temperature behaviour of the electrolyser during operation in the test field. When no hydrogen is produced, the temperature of the electrolyser stack follows the ambient temperature. As soon as the electrolyser is set into operation, the temperature increases. For the day depicted in Figure 5, the temperature rises from 24°C to 46°C during 4.33 hours of operation. As discribed above, this results in an increase in current, and thus an increase in hydrogen production rate over time. Still, the design temperature of 70°C is not reached and hence, a the maximum current does not exceed 11 Ampere.

The time of operation during one day of the electrolyer is varying following the weather conditions (see rules described above). The behaviour of the stack temperature and the current with different hours of operation is compared in Figure 6.

—■— I Electrolyser T Electrolyser

Temperature (°C)

Time Figure 5: Temperature behaviour of electrolyser

T Electrolyser

, Eectrolyser

T Electrolyser

I Electrolyser

Electrolyser

T Electrolyser

It can be seen, that the general temperature behaviour is the same for all days. Longer operation leads to higher operating temperature and thus, to an increase in electrolyser current. A comparison of Figure 6a) and 6b) shows that an interruption of hydrogen production leads to lower operation temperature (4 K) for the same overall time of operation.

Figure 6: Three days of electrolyser operation a) 2.2 hrs (with interruption), b) 2.1 hrs, c) 4.3 hrs

Conclusions

A PV hybrid system has been designed for autonomous power supply of telecommunication equipment. The sizing of the components has been done by life-time cost optimisation using the simulation tool TALCO. Additionally, the rules for operating the system taking have been deduced from the results of simulations over the life-time.

An Energy Management System has been designed in a way it seamlessly uses those rules for its control decisions. Robustness and the possibility of future series production has emphasised.

Operation experience has been gained in a test system set-up in Madrid, Spain. The summer period has been investigated. An analysis of the electrolyser operation shows that less hydrogen is produced than expected from design parameters. The main cause is the temperature behaviour of the electrolyser stack. This problem might be solved by operating the stack at a voltage (up to 2.3 V per cell) during start-up. This would increase heat production and subsequently lead to a higher hydrogen production rate. Technically, this can be done either by a reduction of cells in the stack or by an additional DC/DC converter. The second possibility is favoured as voltage can be adjusted easily to the needs of the hybrid system at different weather conditions. Further work should also be done in including a dynamic model of the electrolyser in the design tool.

While the component prices (especially those hydrogen components which today are not commercially available) heavily influence the overall system profitability, the work shows the practical feasibility to build complex hybrid systems.

Acknowledgements

The work presented has been financed by the European Commission (FIRST, ERK5-CT-1999- 00018).

The authors want to thank all the partners (Project website: www. inta. es/first) for the co­operation during the last four years.