Case Studies of Wind Hydrogen System (part 2)

During 2004 and 2005, four major wind-hydrogen projects have been launched in Europe: the Utsira project in Norway; the HARI project at Loughborough; the promoting unst renewable energy project in Unst, UK; and the RES2H2 project in Greece and Spain.

On the Utsira Island, a wind-hydrogen system was installed to serve 10 households with a peak demand of 45 kW in an autonomous mode. The existing grid connection was kept for emergency situations to avoid the high costs of redundancy. The hydrogen plant consists of a 48 kW alkaline electrolyzer producing 10 Nm3/h H2, a hydrogen compressor for the filling of a 12 m3 storage tank at 200 bars with a 2400 Nm3 H2 capacity, a hydrogen generation set of 55 kW, and a PEM fuel cell of 10 kW nominal power. A 600 kW wind turbine supplies a varying portion of its power to the system feeding the rest of its power to the grid. At maximum load, the electrolyzer and compressor need approximately 54 kW of electrical power.

In the “PURE” project on the Shetland Islands, the wind-hydrogen system is composed of two wind generators of 15 kW power each, a 15 kW advanced alkaline electrolyzer operating

at 55 bars, a 16-cylinder stack of 44 Nm3 H2 capacity at the same pressure, and a 5 kW PEM fuel cell.

A hydrogen plant was integrated to an existing infrastructure of renewables, which included two 25 kW wind turbines, photovoltaics, and two microhydroelectric turbines in the HARI project. The wind turbines are two-bladed, stall-regulated, and pitch overspeed. The hydrogen plant is composed of a 36 kW alkaline electrolyzer, a hydrogen compressor, 48 pressurized hydrogen cylinders of 2856 Nm3 capacity at 13.7 MPa, and two

different fuel cells of 2 and 5 kW nominal power.

The alkaline electrolyzer with a 46-cell stack produces 8 Nm3/h H2 at 2.5 MPa and operates in a 20–100% range of its nominal power. The 3.75 kW single-stage hydrogen compressor has a capacity of 11 Nm3/h for an inlet pressure of 2.5 MPa and an 8:1 compression rate. To reduce the on/off switching cycles of the electrolyzer, which affect its long-term stability and performance, a 20 kWh battery was incorporated. The battery helped to moderate the power supply variations to the electrolyzer.

The mean electrolyzer stack efficiency measured was 75%, whereas the average conversion efficiency of the electrolyzer–battery–compressor system was 49%, including the balance of plant losses. The performance of the electrolyzer decreased gradually over 2 years of operation, with the stack power requirement increasing from 36 to 39 kW and the total power requirement, including the hydrogen purification section and auxiliaries, from a maximum of 43 to 45 kW.

In the frame of the RES2H2 project, a wind-hydrogen system was installed and tested in a wind park near Athens, Greece. The hydrogen system consists of a 25 kW alkaline electrolyzer

producing 5 Nm3/h H2, a hydrogen compressor for filling high-pressure cylinders at 220 bars and metal hydride tanks. The plant and all the auxiliaries are connected to a 500 kW wind turbine, which feeds the rest of its power to the grid. Under variable power input, the electrolyzer rarely reaches its nominal operating temperature of 80°C, and its efficiency varies from 70% to 85% for an electrolysis temperature in the range 45–70°C.

source: (2009) Hydrogen Fuel, Production, Storage & Transportation by Ram B Gupta (ed)

Case Studies of Wind Hydrogen System (part 1)

Water electrolyzers have been designed in the past for continuous or discontinuous operation with grid current converted to DC current. Most of the case studies reported in the literature refer to stand-alone systems. In fact, several of these stand-alone systems have part of their auxiliaries connected to the grid, or use the electrical grid as backup power.

A wind-hydrogen system of 20 kW installed power has been developed, constructed, and optimized at the Fachhochschule Wiesbaden in Germany since 1985. A 20 kW wind energy converter, designed for stand-alone operation, feeds DC to a pressurized alkaline electrolyzer of 20 kW, and the produced hydrogen is used in two gas motor generators of 8 and 4 kW electrical output. The overall energy effi ciency from wind electricity to gas generator electricity is around 15%.

At the University of Stralsund in Germany, a 100 kW wind turbine and a 20 kW alkaline electrolyzer supplying hydrogen at 25 bars to a storage tank of 8 m3 have been in operation for several years. According to the wind speed, the asynchronous wind generator can be operated at 1000 or 1500 rpm producing 20 or 100 kW of electricity, respectively. Both the static and the dynamic behaviors of the wind-hydrogen system were investigated, and an electrolyzer efficiency of approximately 65% with respect to the HHV has been reported.

The Hydrogen Research Institute in Canada has developed and tested a stand-alone renewable energy system composed of a 10 kW wind turbine, a 1 kWpeak photovoltaic array, a 5 kW alkaline electrolyzer, and a 5 kW PEM fuel cell. The components of the system are electrically integrated on a 48 V DC bus.

A small stand-alone, wind-powered hydrogen production plant was designed, constructed, and tested in Italy, at the National Agency for New Technologies, Energy and the Environment. The main aim of the project was to study the control of a wind turbine to produce a smooth power output, the tolerance of an electrolyzer to fluctuating power inputs, and the overall economics of a wind-hydrogen system.

The system was composed of a 5.2 kW wind turbine with a synchronous generator at variable speed, a 2.25 kW electrolyzer, and a 330 Ah battery bank. The variable frequency AC power output from the turbine was rectified and supplied to the electrolyzer. Hydrogen purity remained satisfactory even at low-capacity factors, with oxygen content in hydrogen in the range 0.15–0.35 vol%, although the efficiency of the electrolyzer stack (50%) was lower than state-of-the-art electrolyzers (70%).

Sizing of the Electrolyzer in Wind Hydrogen System

For a given wind power installed, the sizing of the electrolyzer is not trivial. In the case of “stand-alone” systems, a one-to-one approach is often proposed, with the electrolyzer power input being equal to the nominal power output of the wind turbine. In this way, the electrolyzer should be able to retrieve all the wind power in the absence of load. In gridconnected systems, the same approach leads to the choice of an electrolyzer with a power supply equal to the power output of the wind turbine minus a “base load.”

However, the specific capital cost of the electrolyzer being almost equal to the cost of a wind turbine, it is important to take into account the capacity factor of the electrolyzer that will always be smaller than that of the wind turbine.

Electrolyzers are generally current-controlled, which means that a certain DC is imposed according to the desired hydrogen production. In a wind-hydrogen system, the wind turbine

power available for the operation of the electrolyzer is generally known; therefore, the power input should be transformed to a current input.

The voltage–current relation of an electrolyzer is not very simple because it depends on the temperature, pressure, and other construction characteristics. For a given electrolyzer, it is possible to experimentally establish the I–V curve at different temperatures and pressures, and deduce a temperature-dependent current–power curve.

The sizing of the wind turbine, electrolyzer, and auxiliaries is greatly affected by whether the wind-hydrogen system is grid connected or stand alone. The cells can be made in any size, and any number of cells can be stacked in a series depending on the desired output and design point selected. The design point, namely the current density for the cell, is case sensitive and depends on whether the system is operational cost critical or capital cost critical. The cell voltage and therefore the specific power consumption of the electrolyzer depend on the current density according to a I–V curve.

If the capital cost is more important than the operating cost, which depends on electricity price, the optimum current density should be shifted toward the higher range with a lower efficiency. If the operating cost is more significant than the cost of money, the current density should be shifted toward the lower range, or expensive activated electrodes may be used to increase the efficiency. Module material costs usually represent between 44% and 56% of the total material costs in wind-hydrogen system.

Commercial water electrolyzers cover a wide range of hydrogen production rates from 0.001 to 750 Nm3/h. Small hydrogen generators are intended for laboratory use, where hydrogen is often used as a carrier in analytical instruments, whereas large units are used in different fields of the chemical industry.

The actual capital cost of different electrolyzers operating at pressures between atmospheric pressure and 30 bars is presented in Figure above based on offers from manufacturers. There is a wide variation in the specific capital cost of small-size electrolyzers due to the different technical characteristics and options included. The specific capital cost of very small laboratory electrolyzers may exceed 30,000 €/kW, but has not been shown here for simplicity.

There are no large variations in the specific cost of medium-to-large-size electrolyzers, that is, above 200 kW, mainly because there are very few manufacturers around the world that are actually supplying equipment of such a size. The manufacturing process of electrolyzers in wind-hydrogen system is still intensely manual and the prices shown here refer to individual items without taking into account any volume effect.

The specific capital cost of very small laboratory electrolyzers, producing <>

Generally, in wind-hydrogen system the electrolyzer cost increases with hydrogen purity and delivery pressure. In fact, a purification section for the removal of oxygen and the reduction of humidity to a concentration below 10 ppm may represent 30–40% of the total cost.

Advanced electrolyzers actually supply hydrogen at pressures up to 30 bars and there are prototypes with a delivery pressure of up to 100 bars. They are more expensive than atmospheric electrolyzers, but their use decreases the cost of storage: hydrogen may be directly stored at the delivery pressure, and if a compressor is used for filling high-pressure cylinders, it only needs one stage instead of three or four.

source: (2009) Hydrogen Fuel, Production, Storage & Transportation by Ram B Gupta (ed)

History of Wind-Hydrogen System

During the development of electrical generating equipment in the late 1800s, both Europe and America began to experiment with wind power for electrical generation. Among the first to develop wind-powered electrical generators was the Danish professor, Poul La Cour, who worked on wind systems from 1891 to 1908. He also saw the use of hydrogen as a fuel and the use of wind-powered electrical generators to electrolyze hydrogen
and oxygen from water (wind-hydrogen system).

Another early investigator who promoted wind-powered hydrogen production systems was J.B.S. Haldane a British biochemist at Cambridge, England. In 1923, he predicted that England’s energy problems could be solved with a large number of wind generators supplying high voltage power for hydrogen production.

During World War II, Vannovar Bush was the Director of the U.S. Wartime Office of Scientific Research and Development. He was concerned about American fuel reserves and thought that wind generators could be a solution. Percy Thomas was a wind power advocate on the Federal Power Commission, who convinced the Department of the Interior to construct a large prototype wind generator.

In 1951, the House Committee on Interior and Insular Affairs killed this plan. Wind-generated electricity could not compete with coal that was selling for $2.50 per ton or diesel fuel at $0.10 per gallon. The promise of even less expensive electricity that was too cheap to meter from nuclear power plants resulted in the loss of almost all Federal programs to develop wind-powered energy systems.