Applications of Metal Hydride Heat Pumps in Hydrogen Infrastructure

Abstract

Metal hydride heat pumps can provide a temperature lift due to the pressure difference between a hydrogen network and subsequent hydrogen consumers. This temperature lift can energetically enhance low temperature flows by raising them to relevant temperature levels. Pressure and temperature conditions are determined by the hydrogen infrastructure. In this work it was calculated that metal hydride heat pumps with a low-temperature metal hydride can provide approximately 10% of the lower heating value of hydrogen as thermal energy, while MgH2 heat pumps are able to provide 30 % of hydrogens lower heating value as thermal energy, not considering losses. Examined influencing factors on the losses thereby have a significantly higher impact on the thermal output of metal hydride heat pumps with the low-temperature metal hydride than when MgH2 is used. In comparison with an expansion turbine, the lowtemperature metal hydride heat pump can provide thermal energy about three times as high as the electrical energy gained through relaxation of the hydrogen from the same pressure drop, and MgH2 ten times the amount. For the estimation of possibly achievable energy savings, an implementation of metal hydride heat pumps in the entire steel and ammonia production in Germany was calculated in this work. Results show exemplary for the implementation in steel production, that low-temperature metal hydride heat pumps can provide thermal energy in the same order of magnitude as those released during iron reduction in the steel production process. MgH2 heat pumps even achieve twice the values, then available for upgrading heat flows. Next to an integration in the actual hydrogen consuming process, it is also possible to integrate the thermal upgrade or cooling effect into other related energy systems, for example district heating networks. Assuming a temperature lift of 40K, thermal energy provided by low-temperature metal hydride heat pumps corresponds to the heating demand of about 600000 households, while MgH2 heat pumps can provide thermal energy amounts equal to the heating demand of 1.6 million households, assuming a temperature lift of 100 K. On the basis of the results in this work, a decision chart is presented, which simplifies the identification of new application possibilities of metal hydride heat pumps and enables rough estimations of the achievable thermal power.