INTERNATIONAL ENERGY AGENCY
HYDROGEN IMPLEMENTING AGREEMENT
TASK 11: INTEGRATED SYSTEMS
Final report of Subtask A:
Case Studies of
Integrated Hydrogen Energy Systems
Chapter 4 of 11
T. Schucan
Paul Scherrer Institute
Switzerland
Chapter 4
PHOEBUS JÜLICH DEMONSTRATION PLANT
1. PROJECT GOALS
2.
Due to the greatly varying availability and demand, future energy supply systems based on
renewables will require energy storage. At present, the public grid can be used to store
intermittent renewables, as long as the load penetration is low. If, therefore, larger supply
contributions are to be provided from renewables, concepts of energy storage must be
established for the short and long term as important system components of a regenerative
energy supply. Appropriate measures for their realisation must be initiated now.
The overriding objective of the PHOEBUS project is therefore the development and testing of a
solar energy supply system in combination with energy storage. The task is centered on
demonstrating the supply under real solar- and consumption-specific conditions.
3. GENERAL DESCRIPTION OF PROJECT
4.
The PHOEBUS demonstration plant has been designed to provide an autonomous solar
electricity supply to the central library building of the Research Centre Jülich with an installed
capacity of 38 kWel. The major components of the plant in its first phase of operation are:
· A photovoltaic field with four facade and rooftop integrated generators consisting of
monocrystalline modules with an active area of 312 m2, peak power output of 43 kWp, and
an electrical energy output of 29 MWh/yr;
· A pair of DC/DC-converters (5 kW each) for each photovoltaic field, adjusting the voltage to
the level given by the DC-grid which has the actual voltage level of the battery system;
· A system of 110 lead batteries of the OPzS OCSM type with electrolyte recirculation,
designed for a DC-grid voltage of 220 V (200-260 V), and a capacity of 304 kWh, 1380 Ah
over 10 hours;
· A bipolar 21 cell electrolyzer with an active cell area of 2500 cm2, a current of 750 A, a
current density of 3 kA/m2, with 30% KOH solution, 80°C operating temperature, an
operating pressure of 0.7 MPa and 90% efficiency in design load operation at a design power
rating of 26 kW; maximum hydrogen production is 6.5 Nm3/h and maximum oxygen
production is 3.25 Nm3/h;
· A storage system for the product gases, hydrogen (6.5 Nm3/h) and oxygen (3.25 Nm3/h),
leaving the electrolyzer at 0.7 MPa. The hydrogen is compressed to 12 MPa and stored in 18
pressure bottles of 1.4 m3 each with a total geometrical volume of 25 m3 for H2 (3,000 Nm3
H2). The oxygen is compressed to 7 MPa and stored in one high pressure storage unit of 20
m3, sufficient for the whole seasonal storage requirements;
· An alkaline fuel cell system consisting of KOH gas diffusion electrode fuel cell of the
Siemens BZA 4-2 type with a design power output of 6.5 kW at 48 V and 135 A, with a
system efficiency at design load of 63% (LHV of H2) and 70% at 30% partial load.
An overview of the experimental hall with fuel cell, alkaline electrolyzer, gas treatment unit,
highpressure electrolyzer (added in phase 2), battery system (from left to right) and all other
subsystems (except the gas storage vessels), is shown in Picture 4.1.
The central task of this project is to test the required storage system using hydrogen as the
energy carrier. Combining an electrolyzer and a fuel cell with all essential system steps of a
solar electricity supply, the objective is to match the system components and optimise the
overall plant so that energy losses are as low as possible and high system reliability is achieved.
In considering the energy flows and their conversion in the individual plant components it is
important to maximise the energy yield and adapt power supply as efficiently as possible to the
supply task. This requires high system engineering demands on the utilization rates as well as
on the regulation and control of the individual components in a wide power range. On the whole,
the tasks to be fulfilled involve a complex system composed of interlinked components of energy
converters and energy stores integrated between a greatly varying solar radiation input and a
consumer with load fluctuations. Moreover, account must be taken of various requirements to
be met by the electrochemical components with respect to service life, operational reliability
(H2/O2 gases), start-up and shutdown processes as well as exclusion of specific operating
conditions (e.g. low-load ranges diminishing the utilisation rate, avoidance of exhaustive
discharge and gassing charge of the battery, abruptly changing load).
An intelligent strategy must therefore be developed which, at any time of the day and year,
distributes the energy flows to the individual paths and takes account of the boundary conditions
of each system component with respect to its efficiency characteristics and its control and
regulation dynamics.
It can be seen from the above requirements that entirely new standards must be applied to the
system management of such a stand-alone plant, which can only be coped with by a
computerassisted energy management (EM).
The overall system development of energy storage technology relies on plant and systems
engineering as well as open-loop and closed-loop control technology.
3. PERFORMANCE AND OPERATIONAL EXPERIENCE
The general aim was to determine design strengths and weaknesses from the operating
experience, and with the measurement data obtained, verify the models used in simulation
programs, to energetically optimize the plant, to check the operational reliability and to propose
and implement cost-reducing and advanced system modifications.
3.1 Results of a twelve-month operation
The energy balance for an annual scenario (1997) of PHOEBUS is shown in Figure 4.1. The
energy flows and efficiencies measured were entered as annual sums or average annual
efficiencies. The measurement result obtained is basically in agreement with the autonomy
conditions calculated by simulation. It should be noted, however, that the energy driving the two
pneumatic H2/O2 compressors was provided by the compressed-air grid and was not taken into
account energetically, except for the leakage.
Since September 24, 1997, PHOEBUS has operated without the PV converters, i.e. the PV
generators are directly coupled with the busbar and battery, and maximum power point (MPP)
tracking by the converters is omitted. The results show that this step provides a positive balance
compared to previous operation. Although the solar-electric energy yield decreases by 3 - 4%
due to the floating power point (without MPP tracking), the conversion losses of the converter
amount to 9% and the converter investment costs are eliminated. An increase of the battery
voltage by 18 V (9 cells) would even further improve the performance.
A weak point in the storage system continues to be the fuel cell. The operation of the alkaline
fuel cell (AFC) suffered from failures which were inherent in the system: failure of the electrolyte
pumps, disturbances in the control electronics caused by failure of sensors measuring
temperature, current and voltage, and leakages in the gap evaporator led to increased
electrolyte concentrations in the cooling water and required a frequent supply of additional
potassium hydroxide. The stored H2 and O2 gases could thus only be used to a minor extent. In
order not to disturb plant operation by the failure of the fuel cell, the so-called "fuel cell
simulator" switches itself on in this case. This is an AC-DC converter, which takes over the
function of the fuel cell in conformity with power requirements so that autonomous operation
continues to be ensured. On the whole, the fuel cell produced an electric energy of 387 kWh in
the period under investigation. With an average power of 4 kW the fuel cell was thus in operation
for 97 hours.
This low availability was compensated by the "fuel cell simulator" supplying an amount of
energy of 4162 kWh from the grid. In terms of control technology, there was no difference
between the fuel cell and simulator.
The balancing of plant operation was effected by the acquisition and storage of more than 200
data monitoring points measured by the sensors at intervals of one second. This includes
checking of the data for plausibility with several computer programs, reducing them, forming the
corresponding mean and summation values and considering data dropouts. The data are used
to determine and balance the material and electrical energy flows, and power levels and
efficiencies on all paths of the system from solar radiation input up to the consumer. The data
are displayed graphically as time-variation curves or cumulative distributions for arbitrary
periods of time.
An example of the comparison between measured values and simulation results for the
efficiency of the total photovoltaic system (summed over all 4 fields) is given in Figure 4.2.
3.2 Development of new system components
A priority task was the joint development of a fuel cell based on the proton-exchange membrane
technology (PEMFC) for use in PHOEBUS with Sammer, USA. Work for the development,
construction and testing of two 2.5 kW units was largely completed and the units were
incorporated into the PHOEBUS plant. The peripherals (gas supply, cooling, measurement,
electrical installation, control, and power converters) of the two fuel cell stacks were installed.
Initial operation was not satisfactory and further improvements were required which did not lead
to success until the end of the period under investigation. Problems encountered included
membrane ruptures, water management, and channel dimensions. This dashed the hope of
being able to use the new PEMFC for the first time in the winter 1997/98.
The gas storage system for H2 and O2 was operated for the first time without failure. Thanks to
the high operational reliability of the electrolyzer it was possible to fill the gas storage tank for
the
first time at the end of the season (2257 m3), so that the gas quantities required for autonomous
operation were available. The leakage rate (approximately 1 m3/day) was high in the hydrogen
storage tanks at high pressure levels. A total of 19% of the annual production was thus lost. It
was possible to reduce leakage by improving the flange construction. The two compressors
continue to be a weak point of the storage system. Although it was possible to significantly
increase the lifetimes by a new coating procedure for the cylinder bearing surfaces
(approximately 300 h), this means two replacements during one season!
In cooperation with the Central Department of Technology (ZAT) and the Central Electronics
Laboratory (ZEL) of Research Centre Jülich (FZJ), the high-pressure electrolyzer (5 kW, 120
bar) and the two-stage solar thermal metal-hydride compressor (120 bar) were developed as an
alternative to mechanical compression.
The high-pressure electrolyzer is shown in Picture 4.2. It is currently being tested. The
metalhydride compressor was subjected to a thorough practical test within the framework of a
doctoral thesis (K. Bonhoff). This test has been successfully completed.
In summary, the aim of an autonomous supply in automated operation has been achieved. An
efficient control system has been developed by means of well-devised energy management with
battery charge as the control parameter and electrolysis and fuel cell performance as the
variables.
4. SIMULATION AND SYSTEM INTEGRATION
The efficiency, control strategy, and optimum size and configuration of autonomous PHOEBUS
plants are governed by many factors (load structure of electricity and heat requirements, storage
concept, control, costs, site etc.), and they can ultimately only be treated by means of simulation
techniques proven in practical application. Optimization analyses for plant operation (energy
management) and system studies with different plant configurations were carried out with the aid
of simulation programs validated by measurements in PHOEBUS. This has led to simplified and
more efficient PHOEBUS concepts incorporating reduced expenditure on electrical power
conditioning (omission of the three power converters) and omission of mechanical compression
by the use of a high-pressure electrolyzer.
The stability and controllability of direct coupling of the PV generator with the battery,
electrolyzer, and fuel cell have been thoroughly investigated and feasibility was demonstrated by
simulation. The efficiency of the overall plant can be improved from 54% to 65% and an
appreciable reduction in construction costs may also be expected by the omission of the
converters.
The aim of a further investigation of different system concepts is to explore the possibilities of
integrating regenerative energy converters in a combined heat and power package of
decentralised energy supply plants and to present corresponding systems.
Initial results have shown that a number of promising system variants based on the PHOEBUS
concept and using the simulation tools are conceivable. These utilize the hydrogen storage
system (electrolyzer/fuel cell) presented in order to either realize a purely electrical or an
electrical and thermal energy supply. It is very advantageous to use another renewable source
of energy (ambient heat) and incorporate it in the PHOEBUS system with the aid of an
electricmotor- operated heat pump circuit for heating or cooling purposes.
On the input side of the energy supply system, wind energy also offers itself as an alternative to
or can be used in combination with PV energy. Depending on the site, a hybrid operation of both
energy sources leads to a homogenization of the annual variation in energy generation. This
generally leads to a reduction in storage requirements compared to operation with a single
source, and long-time storage becomes entirely superfluous if energy surpluses are abandoned.
Such hybrid systems also offer the possibility of adapting oneself more or less to the
consumption time-variation curve by selectively choosing the nominal power of each plant with
its outputs. This will then require, for example, a correspondingly higher wind generator power
for covering the electric and heating energy requirements or a higher PV generator power for
meeting the electric and cooling energy requirements in the case of a strictly demand-oriented
heat-power/coolant coupling.
5. FUTURE PLANS
With the aid of largely validated simulation programs, optimization analyses for plant operation
and plant configuration were carried out and two advanced concepts, PHOEBUS-2 and
PHOEBUS-3, developed.
The PHOEBUS-2 concept involves a greatly reduced system expenditure and thus a cost
reduction of the entire PHOEBUS plant. This can be mainly achieved by directly coupling the PV
generator with the battery, electrolyzer, and fuel cell. In this way, the expenditure and plant losses
can be reduced by omitting the power converters. The use of a high-pressure
electrolyzer makes the mechanical compression of the H2 and O2 product gases superfluous.
This leads to a further reduction of the losses on the storage path so that the overall plant
efficiency can be reduced by 10% to 15% compared to the present-day plant. Initial successes
in the practical application of new or extended systems seem to be achievable with this
procedure.
PHOEBUS-3 was conceived within the framework of a doctoral thesis (Ch. Meurer). It has been
investigated whether and how an electrical and thermal supply in conformity with demand and
purely based on renewable energy can be achieved with a modified system technology. It was
shown in simulation calculations that the use of ambient heat (installation of heat pumps) makes
it possible not only to uncouple energy supply (PV and/or wind energy) from energy demand in
terms of time and power, but also to uncouple the electrical from the thermal energy
requirements (heating) in terms of time using the same storage system (electrolyzer-H2-fuel
cell). Wind and photovoltaic generators in hybrid operation prove to be particularly advantageous
due to their complementary annual variation. The storage requirements are considerably
reduced due to the relatively uniform annual energy production.
6. CONCLUSIONS
PHOEBUS Jülich demonstrates that an electrical energy supply with purely renewable energy
without connection to the public grid is basically possible. Plant operation is fully automated. The
control strategy of the plant has been constantly improved so that a developed energy
management system is now available as system software. The control system optimally
distributes the energy flows and completely ensures the functionality of the plant by means of
monitoring systems.
It has been possible to identify weak points of the plant by simulation and measured data
evaluation: The optimum system management strategies were determined for individual
components using sensitivity analyses.
The weak point of the storage system continues to be the fuel cell. Despite great efforts in
developing the PEM fuel cell, it has not been possible to ensure operational reliability over
prolonged periods. Worldwide efforts at making these fuel cells marketable in terms of
technology and costs would also mean a breakthrough for the hydrogen (or fuel) storage system
and its multiple application possibilities.
As stated above, the long-term hydrogen storage system proves to be the central unit of every
system variant to decouple energy supply and demand in terms of time and power, but also to
decouple electric and thermal energy requirements in terms of time using the same storage
system. This is above all a clear advantage compared to conventional CHP plants. Current
investigations are also aimed at finding cost-optimized systems in addition to those technically
optimized. This will primarily be achievable by minimizing the storage system.
The know-how required for the above tasks can be utilized in further system studies and in the
conceptual design and qualified handling of new tasks and projects.
The experience of PHOEBUS, including the measurement evaluation of the plant and the results
of system engineering studies obtained for the conceptual design of renewable energy supply
systems, can open up new paths for the development or improvement of equipment and systems
technology (PEM fuel cell, pressure-type electrolyzer, metal-hydride compressor,
metal-hydride storage, H2 safety technology, H2/O2 gas treatment, system modelling, energy
management, control and regulation technology).
Plant costs can be further decreased with standardized system-engineering solutions and with
the market introduction of electrochemical energy converters (electrolyzer or fuel cell). The
opportunities for near-term market introduction are: wherever a connection to the public grid is
not economically efficient (due to extremely long distances or insufficient load compatibility);
where decentralised CHP plants can uncouple power and heat; reduced emissions; lower fuel
costs (including transport and storage); or a specific peak load in the grid must be
accommodated.