Verlag des Forschungszentrums Jülich
JUEL-4049
Photovoltaic and wind energy converter is considered as energy converter and energy is
supplied to a microwave repeater station. The hourly average solar radiation and wind speed
for whole year as well as the load profile of the microwave repeater station is considered as
input data for present optimisation method. In this optimisation method, the different
components have different lifetime. It is expressed in different units. For example, the REC
lifetime is expressed in year where as the battery life is expressed in füll cycles and the
electrolyser and fuel cell's lifetime is expressed in terms of operating hours. In present
optimisation method the battery capacity is considered in such a way that the lifetime of the
system becomes integer multiple of the battery lifetime. The life of the electrolyser, the fuel
cell and the tank does not end at the end ofthe system's lifetime. The cost of the components
at the end of the system's lifetime is calculated an the basis of the remaining lifetime of the
components, general escalation and the discount rate.
Normally, the battery is not optimised in the renewable energy system. It is more usual to
consider a predetermined size ofthe battery an the basis of the number of days of autonomy.
In the present optimisation method, no such predetermined battery capacity is assumed. The
battery size is optimised considering the economic parameters such as the discount rate and
the inflation rate. Any Optimum battery capacity cannot be obtained without considering
these two parameters. The Optimum battery capacity depends an the ratio of the escalation
rate and the discount rate . It also depends an mismatch between the load demand and the
energy supply from the REC. The supply cannot be made significantly more reliable by
increasing the battery capacity beyond the Optimum size.
Considering the Optimum size of the battery, the other three cost intensive components
(PV/WEC, electrolyser, tank) in the System are optimised simultaneously . During
optimisation the lifetime of the components are also taken into consideration. The REC size
is incremented in small steps. Due to the increase in the size ofthe converter, the total annual
energy production also increases. A fraction of the increased energy is used by the load and
the rest is wasted due to insufficient storage capacity. The used energy part always decreases
with an increase in the REC size, resulting in an increase in the effective cost of the energy .
For each REC size, the effective energy cost is compared with the storage cost and the cost
optimum REC size is determined where the effective energy cost becomes equal to the unit
energy storage cost.
The tank volume and the electrolyser size are optimised simultaneously . For each REC size,
the optimum sizes of the tank and the electrolyser are determined an the basis of the surplus
and the deficit energies in the system. The REC sizes for which the system is incapable of
providing the total annual energy demand, the electrolyser and the tank sizes are determined
an the basis of the surplus energy in the system . For the other REC sizes the electrolyser and
the tank are optimised an the basis of the deficit energy in the system. Thus for all the REC
sizes, an optimum tank size and an optimum electrolyser size are obtained from the present
method for a particular load and weather data. Among these optimum sizes, the size
corresponding to the optimum photovoltaic size represents the cost optimum size for the
electrolyser and the tank.
In a PV-WEC hybrid system, both the PV and the WEC serve as energy converters . At the
different photovoltaic sizes, different sets of optimum sizes of the WECS, electrolyser and
tank are obtained. Out of these optimum sets, the most cost-effective set has to be selected.
This is done an the basis of the total investment cost of each set and a set requiring the
minimum investment cost is then picked .
There are different ways of storing hydrogen in long-term energy storage . Similarly, there
are many options to retrieve the energy out of hydrogen. It is possible to store hydrogen at
low pressure, which demands high tank volume . By introducing the compressor in the
system the tank volume can be reduced which would reduce the investment cost for the tank
but an extra cost is introduced for compressor . A small system, which is able to supply
energy around 2000 kWh per annum from the photovoltaic is cost-effective if the
compressor is included in the system. The long-term storage consists of two different loopshydrogen
loop and the oxygen loop. The hydrogen loop is the main energy storage loop. The
other loop may be totally eliminated by using air. However, the use of the air in the fuel cell
would reduce its efficiency. As a result of that, more tank volume is required which would
increase the investment cost for tank. Considering these effects it is determined that the longterm
storage only with hydrogen loop is cost-effective. The cost effectiveness would
increase with the increase in the size of the system.
An efficient energy management strategy is essential to handle surplus and deficit energies
in the system . The state of charge of the battery plays the main role in the energy
management strategy. The entire range of the SOC is divided into different parts . The
operation of the different components depends an the SOC level. In this thesis, five-step
energy management in PHOEBUS (Barthels et al., 1996) has been reduced to a three-step
energy management. Four different options of energy management are taken into
consideration and an efficient energy management strategy is found.
Hydrogen as storage medium in a renewable energy system increases the reliability of
supply. The same purpose can be achieved by a hybrid system in which a diesel generator is
used in addition to the battery. Thus, it is very important to answer where a hydrogen based
storage system would be useful. The diesel-based system has the advantage of needing lower
investment costs. The other advantage of the hybrid system is that its technology is well
established and widespread . However, the main disadvantage is that it needs the fuel to be
supplied for the operation of the generator whereas the hydrogen-based storage has an
advantage over the diesel generator is of not requiring any fuel supply. Thus, the
transportation costs of the fuel are eliminated fully. The diesel based renewable energy
system is liable to produce a big Surplus energy in some locations, which is wasted.
However, in a diesel-based hybrid system, the Surplus energy in the system is not stored. The
sites are classified depending an the seasonal energy variation and the total fuel cost. The
total fuel cost at the location of the application includes the transportation cost. The total
lifetime costs ofthe hydrogen-based system are compared with the lifetime cost of the diesel
generator system . The lifetime cost ofthe hydrogen storage is calculated without energy cost
because the surplus energy, which is used in the electrolyser, would not be recovered by
introducing a diesel generator. Comparing the lifetime cost of both the Systems, a boundary
curve is calculated which gives the boundary fuel cost for different seasonal solar/wind
energy difference. On the same plot, the different sites are denoted by the different points.
The boundary curve determines where the hydrogen-based system is cost-effective . The
applications at a highly isolated region (polar region) the hydrogen-based system is costeffective
in spite of the high fuel cell cost. In the future as the cost comes down the
possibility of the cost-effective application of the hydrogen based renewable energy supply
system would be increased. For a PV-WEC hybrid system, the cost-effective region is
determined by a boundary plane. The important factor for such system is the overlapping
between the annual profile of the solar radiation and the wind energy . With the increase in
the overlapping more and more sites would become cost-effective for the hydrogen-based
system.
Ghosh, Prakash Chandra
Cost optimisation of a self-sufficient hydrogen based energy supply system
XII, 122 S., 2003
The Overall goal of this thesis has been to develop an optimisation method for a hydrogenbased
renewable energy supply system . The cost-effective application of such a system is
also analysed. The main cost intensive components, used in the System are optimised an the
basis of the lifetime and the investment cost of the different components . The main cost
intensive components are the renewable energy converter (REC), the battery, the
electrolyser, the gas tank and the fuel cell. The sizes of the first four components are
interdependent. A comparative analysis of hydrogen and diesel generator based renewable
energy supply system is performed.
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