J. P. Vanhanen

Electrolyser-Metal Hydride-Fuel Cell System for seasonal energy storage

Abstract A small-scale seasonal energy storage system, comprising an electrolyser, metal hydride hydrogen store and fuel cell, has been studied. According to the feasibility study, solid polymer electrolysers and fuel cells are the best options for the electrolyser-metal hydride-fuel cell energy storage systems. A round-trip efficiency of 30% has already been demonstrated, and the next target is to reach a round-trip efficiency close to 40%. The electrolysermetal hydride-fuel cell systems are suitable for small-scale self-sufficient applications in which high volumetric capacity is needed and safety aspects are appreciated.

Development of a self-sufficient solar-hydrogen energy system

Abstract This paper describes the status of a photovoltaic hydrogen energy system development project at Helsinki University of Technology at the end of June 1992. The objective of the project is to demonstrate the technical feasibility of a 100% self-sufficient energy system based on solar photovoltaics (PV) and hydrogen technology. Basically, PV electricity is used to produce electrolytic hydrogen, which is stored over the season to be converted back to electricity in a fuel cell. The pilot plant has been designed for a 1–2 kWh day−1 constant electric load in the climate of Helsinki (60°N). The work so far has included component and subsystem testing, as well as optimization of the total system and its control through comprehensive numerical modelling. Experimental results are given for the electrolyser performance as well as for a 1 month operation of the hydrogen production subsystem. The numerical simulation shows excellent agreement with measurements and is used to predict the pilot plant performance over a 1 year time period.

Combined hydrogen compressing and heat transforming through metal hydrides

Abstract The feasibility of combined metal hydride compressor and heat pump (MHCHP) has been studied through characterising of various synthesised and commercial alloys. The operation of a two-stage metal hydride compressor has been demonstrated: hydrogen pressure was increased from 12-18 bar up to 85-110 bar by utilising very narrow temperature interval (20-60°C). By adding a third alloy, TiCrMn 0.55 Fe 0.30 V 0.15 , the hydrogen pressure close to 200 bar could be achieved. The heat pump cycle was also demonstrated by using the same alloys as in the two-stage compressor. A MHCHP would be very useful for self-sufficient solar energy systems: for example in a PV-hydrogen system, a three-stage compressor can increase the storage pressure of hydrogen from 30 bar up to 200 bar, and the heat pump can reduce the power of photovoltaics and the size of the hydrogen storage by more than 10%. 1999 International Association for Hydrogen Energy.

Computational approaches for improving seasonal storage systems based on hydrogen technologies

A comprehensive mathematical model for the hydrogen storage system is derived in order to get a closer insight into total system efficiency and different loss mechanisms. The hydrogen production, storage and conversion sub-systems are studied in detail and respective models with illustrative case examples are presented. The hydrogen balance of the pressurised hydrogen-storage vessel is described by a first-order differential equation, while the electrochemical performance of the electrolyser and the fuel cell are described by the voltage and current efficiencies. The overall performance of the hydrogen production and conversion sub-systems is described by production and conversion efficiencies which also consider parasitic losses. The electrochemical characteristics are the most significant factors influencing total system efficiency, but parasitic losses, such as the power consumption of the process control and gas handling system, or hydrogen gas losses, may also play a significant role in small-scale applications.

Operation experiences of a phosphoric acid fuel cell in a solar hydrogen energy system

A phosphoric acid fuel cell (PAFC) has been connected to a small-scale autonomous solar hydrogen energy system. The performance of the fuel cell has been studied by using its current-voltage curves and energy balance calculations. According to the operation experiences, the advantages of the PAFC are its ability to use air as the oxidant, compact design without electrolyte loop, and high value waste heat. On the other hand, the disadvantages are the need of pre-heating and the open-end stack construction which cause significant energetic losses and decrease the operational efficiency.

Simulation of solar hydrogen energy systems

Abstract Solar hydrogen is a promising long-term global energy option for the post-fossil fuel era. On the other hand, solar hydrogen may have already found an early commercial application in the form of seasonal energy storage for remote stand-alone photovoltaic (PV) applications. In a stand-alone solar hydrogen energy system, the photovoltaic array is coupled with an electrolyser to produce H2 which is stored to be later converted back to electricity in a fuel cell. The system setup comprises several subsystems which have to be controlled in an optimal way. Numerical simulations are used to get a closer insight into the transient response behavior of these elegant, but rather complicated systems during variable insolation conditions and to estimate the overall system performance accurately over extensive periods of time. The simulations are performed with the H2PHOTO program which has been successfully used for the design of a solar hydrogen pilot plant. It has also shown good accuracy against experimental data.

AB2 metal hydrides for high-pressure and narrow temperature interval applications

Abstract AB2-based metal hydrides have been studied in order to find high-capacity, low-hysteresis alloy–hydrogen systems for high-pressure applications with strict thermal boundary conditions. TiCrMn1−3xFe2xVx (x=0, 0.05, 0.1, 0.15 or 0.2) and Ti1−yZry(CrzMn1−z)2 (y=0.05 or 0.15 and z=0.5 or 0.6) alloys have been synthesized and characterised by XRD, ICP spectrometry and volumetric PCI measurements. In addition, the PCIs of two commercial (GfE) alloys, Hydralloy C2 and Hydralloy C0, have been measured and a PDSC study on Hydralloy C2 has been performed, in order to assess the feasibility of their basic hydriding properties for narrow temperature interval applications. In the Fe and V containing alloy–hydrogen systems, hysteresis can be overcome at the cost of reduced hydriding capacity, while in the Zr-containing hydrides, at the temperatures of this study (−80 to 60°C), hysteresis is not completely eliminated but the hydriding capacity remains good also at high temperatures. The interplay between these properties of hydrides is discussed, as well as the role of materials characteristics in specially constrained applications.

Feasibility study of a metal hydride hydrogen store for a self-sufficient solar hydrogen energy system

Abstract The feasibility of using metal hydride hydrogen storage in a self-sufficient solar hydrogen energy system is studied. Several potential commercial and non-commercial metal hydrides are considered to find a material having a low Δ H value, a low hysteresis effect, gentle P - C - T , plateau slopes and a high hydrogen storage capacity. A 1 N m 3 metal hydride container employing a commercial Hydralloy C15 metal hydride with the proper P - C - T curves is analysed in more detail. As the thermal behaviour of the container is crucial in our application, steady-state and time-dependent thermal properties of the container are measured and the respective models are derived. The metal hydride container is also tested under realistic conditions to get further operational experience on its technical feasibility. Based on this study, low-temperature metal hydrides seem to be technically and economically feasible for small-scale self-sufficient solar hydrogen systems in which high volumetric energy density is needed due to limited space.

Control of battery backed photovoltaic hydrogen production

Abstract A battery backed photovoltaic (PV) hydrogen production system shows several advantages over non-battery systems, especially in cases in which the PV array is not totally dedicated to hydrogen production. The use of battery state-of-charge (SOC) and time of the day limits as control parameters to optimize hydrogen production in a battery backed PV-electrolysis system may increase the annual H2 production by more than 10%. Through shunt resistor regulation of the electrolyzer current, a high system efficiency may be maintained even if the power of the PV array well exceeds that of the electrolyzer.

Metal Hydrides for a Compressor With Strict Boundary Conditions

A potential application of metal hydrides in self-sufficient solar hydrogen energy systems is to compress the electrolytically produced hydrogen with a metal hydride compressor (MHC), in order to reduce the volume of the seasonal gas storage. Compared to the use of metal hydrides as the hydrogen storage media, far less of often rare and expensive alloy materials are needed. In addition, MHC integrates very well into the overall energy system if the waste heat of the electrolyzer can be used as the heat source for the thermally driven compressor.