System modelling for hybrid solar hydrogen generation and solar heating configurations for domestic application

Abstract

Hydrogen generation has the potential to deliver an environmentally friendly, low-cost, and renewable energy source. One promising generation method is solar water splitting via a photoelectrochemical (PEC) reaction as an alternative to a combined photovoltaic-electrolyser system. Although PEC technology shows potential, the efficiency of this technology is currently limited by thermodynamics and technical issues in implementation. The development of novel materials is one route for improvements in PEC system efficiencies. In particular, with multiple band-gap electrodes, the thermodynamic efficiency, and so the overall generated hydrogen quantity, can be increased. In the case of applications where there are heating requirements beyond the need to generate hydrogen, there are further options for extracting energy from the solar resource. Longer wavelength radiation not used by the PEC system may be available for use. Just as it is possible to have a photovoltaic-thermal (PV/T) hybrid system which generates both electricity and heat, a PEC unit may also be combined with a solar thermal unit as a hybrid PEC/T system. This combined heat and power (CHP) system will deliver heat directly and also both heat and power through the use of the hydrogen as a fuel in, for instance, a fuel cell. Despite the promise of PEC technology, there is little research in modelling and system simulation and especially for hybrid systems. Systems' modelling is a prerequisite for optimal design, especially for the design and exploration of novel configurations. A system model of a dwelling, with varying heat and power demands, together with a hybrid PEC/T system for meeting these demands, has been developed and implemented in Matrix Laboratory (MATLAB). The full system integrates a PEC unit for hydrogen generation, a solar thermal unit, a proton exchange membrane (PEM) fuel cell, a hydrogen storage tank, and a buffer tank for heat storage. The model has been evaluated through a case study consisting of a typical three-person household in the UK. The aim of the case study is to investigate present and near-future capabilities of renewable energy supply and CO2 emission reduction subject to the UK building energy regulations. Results show that single band-gap photo-electrode materials are not able to meet the energy demands of the household adequately if the demand includes power and both space and hot water heating. However, with novel multiple band-gap electrodes, in a hybrid CHP system, the system efficiency can be significantly increased, and we demonstrate the potential to help meet the comprehensive demands of a typical household through the development of novel materials for PEC reactions.