Hydrogen Private

Size of little fuel cells technologies and internal combustion engines are today implemented. They showed excellent performance and safety in hydrogen city buses as well as in private cars. For example, BMW has successfully demonstrated hydrogen private cars (e.g. BMW 753i) for the past five or more years, since 1990 [51]. Hydrogen fuel in the vehicle can be stored in gaseous form (compressed gas), as a liquid (-253°C), or in solid media. A cryogenic hydrogen (LH2) storage for vehicles offers great advantages compared with compressed gaseous storage, because it offers the highest density per volume. High-pressure hydrogen is stored in a thick-walled tank made of high strength material to ensure durability. Meanwhile, liquid hydrogen is stored in a double walled vessel with insulation, sandwiched between the walls. The study considers a hydrogen private car where hydrogen is stored in liquid form (LH2).

3.6.1.1 System Description

Fig. 3.12 shows the arrangement of the main components of the BMW hydrogen private car. It is a private car driven by an internal combustion engine (ICE) modified from standard 6-cylinder gasoline engine, 2.5-5 litre, 80-140 kW, and with the driving range of 400 km [218]. The car stores hydrogen in liquid form (LH2) in a cryo-tank at a temperature of -253°C, and a pressure of about 0.5 MPa. The LH2 is transported to the engine compartment through a liquid line, and is heated in a central exchanger by engine water up to - 1°C. The GH2 is then fed to a central, electrically operated supply valve with electronic control that injects the required fuel to each cylinder inlet port in sequence. The engine runs under lean conditions for all driving conditions, giving high efficiency and very low NOx emissions.

Auxiiiary-Power-Urirl

Lox Lh2 Engines
Figure 3.12 Arrangement of the main components of the BMW 735i with ICE [218] 3.6.1.2 The LH2 storage

The study carried out by Directed Technology Inc (DTI) concluded that the most risk is contributed by the hydrogen tank. Therefore, the study is focused on the LH2 tank.

Linde Tank Technologie

Figure 3.13 The LH2 tank (Linde) installed in a BMW hydrogen car[219]

pnçin™ rçgiilatpr sliul utr VHVt mjil-çifl Vil*» •flfrty bdil.ùff lyjlim tupport p»t ■14 ■..■

Figure 3.13 The LH2 tank (Linde) installed in a BMW hydrogen car[219]

The LH2 fuel tank (Fig. 3.13) is installed safely in the car trunk (back side of the passenger compartment) so that any release of gaseous hydrogen is directed away from the driver or passenger compartment of the vehicle. The fuel tank is mounted in a location to minimize damage from collision to the fuel tank itself and its accessories. The liquid hydrogen fuel tank is equipped with a hydrogen detection system that sounds an audible alarm if the level of gaseous hydrogen exceeds 20 % of the lower flammability limit.

Table 3-4 Most important capacities and dimensions of the LH2 storage in a car

H2 Storage/line Dimension_Capacity_

1. LH2 tank L=0.0m, D=0.4m, V=0.15m3 150 l (6 kg of LH2)

Source: Linde AG; (*) estimated value 3.6.2 Hydrogen for Household Applications

Hydrogen supplied to buildings (e.g. residential) can be used to provide energy in the form of heat and electricity by using fuel cells as combined heat and power (FC-CHP) generators. Two CHP options have been tested so far in Germany, i.e. CHP-based natural gas and CHP-based pure hydrogen (see Section 2.4.3). The fuel option required for the first CHP was natural gas to be transformed into hydrogen in a separate reformer unit. Based on this, the Vaillant GmbH is currently developing systems for individual households. Hydrogen is reformed from natural gas, which is already distributed over the existing infrastructure. Later on, it is conceivable to distribute pure hydrogen through the same pipelines. On the other hand, the local power company Hamburg (HEW) demonstrated a CHP supplying a clean energy to a whole block of buildings working on pure hydrogen for 3 years (1997-2000), as shown in Fig. 3.14. The last FC-CHP option was interesting to be considered in the study because of involving large amounts of hydrogen storage situated in a residential area. The plant is located in Lysersrasse, Hamburg-Bahrenfeld.

3.6.2.1 System Description

Two major German utilities based in Hamburg, "Hamburgische Electricitâts-Werke AG" (electricity and district heating), and "Hamburger Gaswerke GmbH" (gas), have founded a joint venture (ARGE) to build and operate two phosphoric-acid fuel cells (PAFC) in urban surroundings. One fuel cell is fueled by natural gas and the other by hydrogen. The performance of each cell was 200 KWel and 220 KWth. In combination with an existing heat pump system, the fuel cells provide electricity and low-temperature district heating to residential buildings [79]. A hydrogen-fed CHP (Fig. 3.14) was installed as a demonstration project funded as part of the EU's EQHHPP (Euro-Québec Hydro-Hydrogen Pilot Project) in 1997. The objectives of EQHHPP were to demonstrate a hydrogen fueled energy system in urban surroundings. The focus was not only on the technical and operational aspects to meet public utility demands, but also on questions of public acceptance and legal aspects of transporting and storing hydrogen within a densely populated European city.

The fuel cell system designed for CHP applications primarily consists of a liquid hydrogen storage tank, ambient evaporator, fuel cell system, and heat & power station, as shown in Fig. 3.15. Each fuel cell system consists of two primary subsystems: the fuel cell stack that generates direct current electricity; and the power conditioner that processes the electric energy into alternating current or regulated direct current.

Figure 3.14 The FC-CHP plant at Lyserstrasse in Hamburg-Bahrenfeld[166]

Household demand

AC,--""' ^^ DC

Control systems

Heat exchanger for room heating

Evaporator

FUEL CELL UNITS

Evaporator

Heat exchanger for room heating

FUEL CELL UNITS

Cold water

Figure 3.15 Block diagram of a hydrogen fuelled FC-CHP for household applications

For safety reasons, the hydrogen storage facility required the acceptance of the residents because it is located in an urban area. The fuel cell system was operated under the surveillance of the local safety authority (Amt für Arbeitsschutz, AfA). The safety check determined that the biggest safety hazard of the entire fuel cell unit was the pressure vessel containing water and steam at an operating pressure of approx. 1 MPa. Therefore the fuel cell unit had to be analyzed according to the pressure vessel ordinance [79]. Meanwhile, permission for the hydrogen tank was applied under the Federal Immision Act (Bundes-Immissionsschutzgesetz, BImSchG) through a full process with public participation. The Federal Institute for Materials Research and Testing (Bundesanstalt für Materialforschung und Pruefung, BAM, Berlin) provided a safety report for the preliminary testing of a tank and evaporator plant for liquefied hydrogen [79].

3.6.2.2 The LH2 Storage

The qualitative safety evaluation of the CHP-based pure hydrogen plant showed that the LH2 storage was the largest contribution to the overall risk. Therefore the study was focused on the LH2 storage and its environments. The hydrogen infrastructure required for the system consists of a storage tank and refueling applications for liquid hydrogen (LH2) and an evaporator for the fuel preparation.

Evaporator
Figure 3.16 Simplified P&ID of the LH2 tank at the CHP plant [223]

Fig. 3.16 shows the simplified P&I diagram of the LH2 tank that consists of two concentric walls (envelopes). It is used to store liquid hydrogen at low temperatures. The internal wall is made from stainless steel, and the external wall from carbon steel. These two envelopes are separated by super-insulation thermal (fire-resistant rock wool and aluminium) and the interspace is under a guaranteed vacuum of 1.33 x 10-9 MPa The tank has dimensions of 13.8 m of height, 3.1 m of external diameter, and internal volume (geometric) of 66.3 m3. It has a capacity of 4282 kg of LH2, consisting of 90% of liquid and 10% of vapour. The operating condition of the tank is at a pressure of 1.2 MPa, and a temperature of -253°C.

The tank contains liquid hydrogen with a gas phase on top of the liquid phase. The pressure of the gas phase is controlled by means of a pressure regulator (PCV-3), functioning like a pressure reducer, and a pressure build-up circuit. Withdrawal of hydrogen is made by opening the liquid hydrogen drain valve at the tank bottom directed towards the evaporator to be vaporized by the heat from ambient air. In case hydrogen is not used, the pressure goes up slowly in the tank because of the natural heat entries through the insulation and equipment of the tank. The increased tank pressure is monitored and manual degasifications are generally carried out by the operator, before the pressure reaches the pressure of opening of the valves. The economizer (PCV-4) valve remains closed if its set point pressure is higher than the tank pressure. It sends gaseous hydrogen to the utilization circuit when the tank pressure reaches its set point.

3.6.2.2.1 Filling Circuit

The tank is supplied periodically by trailer trucks of 53000 l of LH2 supplied from the nearest production plant. Filling of the tank is carried out through a double wall flexible hose (like the tank). At the tank side, it is equipped with a non-return valve (Car1) protecting automatically from any leakage of liquid hydrogen. The filling can be done in the liquid phase through valve V-2 (to increase tank pressure) and/or gas phase through valve V-1 (to reduce tank pressure).

The tank filling is controlled by two operators, who observe the liquid level on the level indicator. The filling is stopped manually if the high level is reached. To protect the tank against overfilling, the high level is measured continuously and connected to an alarm via level switch, LSHL. If the high level is reached, firstly an alarm light signal and audible alarm are activated. Moreover, the tank is also equipped with an overfilling gauge (liquid hydrogen detection per bulb hydrogen). This makes it possible to avoid all overfilling of the tank, by automatically closing the pneumatic valve of the truck using its transmitted high level signal (LSH).

3.6.2.2.2 Pressure Building and Economizer Circuit

In order to maintain the tank pressure the tank is equipped with a pressure build-up circuit and economizer. The circuit vaporizes liquid hydrogen from the bottom of the tank and sends hydrogen in the gas phase to the tank (top). Operation of the circuit is controlled using the pressure regulator, PCV-3. If the pressure in the tank is low, the circuit is working. This circuit is protected against overpressure by a relief valve (SV-3). It is installed between the two isolating valves V-15 and PCV-3. In case hydrogen is not consumed, the pressure of the gas phase in the tank tends to increase. When this happens the pressure regulator PCV-4 (Economizer devices) sends the hydrogen gas from the tank to the circuit of utilization.

3.6.2.2.3 Pressure Relief Devices

Two sets of safety valves and rupture discs are installed in parallel to protect the tank against overpressure. One of two safety valves (SV-1, SV-2) is operated exchangeable at a relative pressure of 1.32 MPa. In case the safety valve does not provide relief, one of two rupture discs (RD-1, RD-2) will burs at a bursting pressure of 1.56 MPa. They are used to evacuate all the hydrogen in the event of loss of vacuum of the tank inter-space between two walls, or in case of fire on all the wall surface of 600°C. One set serve as backup, allows operation one or the other of the 2 sets.

3.6.2.2.4 Withdrawal Circuits

Hydrogen is withdrawn from the tank in the vapour phase and used to supply a fuel cell for household applications (FC-CHP). An automatic-close valve (PCV-1) is placed closely to the tank. It allows closing of the hydrogen supply to the utilization circuit in the event of abnormal pressure drop in this circuit. This valve is regulated by the pressure controller (PS) having a signal pressure of 0.1-0.2 MPa less than the operational pressure. Another automatic closing valve (PCV-2) is placed downstream of the evaporator allows closing of the utilization circuit in the event of very low pressure in the tank. The valve is regulated by a temperature detector (TSL), with the set point of -40°C.

3.6.2.2.5 Instrumentation and Control

A pressure indicator (PI) is installed to measure and to indicate the tank pressure. The tank level is measured using a level gauge (differential pressure) for indicating the liquid hydrogen level in the tank. A pressure switch (PSHL) is connected to the gas phase of the tank. It is used to actuate a high-pressure alarm and a low-pressure alarm. A level switch (LSHL) associated with measuring of the liquid level by differential pressure (LI). The LSHL is connected to a high-level alarm, when the high level is reached then the light and sound alarm are activated. In order to protect the tank against overfilling, an overfill detector (Mb) is installed. It makes possible to avoid overfilling of the tank. In the event of overfilling, the LSH actuates a visual/sound alarm, and an automatic stop of the transfer (closing of the pneumatic valve of the supply truck).

3.6.2.2.6 Stack for Evacuation ofHydrogen

The installation is equipped with a stack, which is constructed from stainless steel tube of diameter of 114 mm and a height of 20m. It is placed close to the tank. The stack is used to vent all hydrogen release from the two valves and the two rupture discs, the circuit of venting of the tank, and the purging of the filling terminal. A tube is located partly low to evacuate rain water which can be accumulated.

3.6.2.2.7 Evaporator (heat exchanger)

The evaporator vaporizes the liquid hydrogen into gaseous forms by heat from the air. It consists of tubes made of aluminium alloy coated internal and external fins. It has a capacity of 4282 kg, with the following characteristics: surface area of 72 m2, flow rate of 24 kg/h, utilization maximum pressure of 3 MPa

Table 3-5 The most important capacity and dimension of the LH2 storage at CHP plant Components_Dimension_Capacity_

1. LH2 tank Vertical cryogenic tank, H=13.8 m, 4200 kg of LH2;

D=3.1 m, V=68 m3, Vuseful=64.5 m3 Pmax = 12 bar, T=-250°C

2. Liquid lines Diameter of 3 inch (*)

Source: TUHH, MVE, Air Liquide; (*) estimated value

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