DEMAND/SUPPLY MATCHING

1. INTRODUCTION

The number of PV panels required to power the autonomous dwelling and the size of the hydrogen storage component is a key element of the feasibility of this project and has been determined by using the "Merit" software developed by the ESRU department at the University of Strathclyde.

The "Merit" software matches different types of electricity demand with different types of renewable energy supply and backup system by using climate and demand side data. To run a simulation, we are required to select a climate and a simulation period, select a demand profile and its consumption over the simulation period. We then define the type and size of the energy supply and backup system. Finally, a demand/supply matching simulation is carried out. The size of the energy supply and backup system is adjusted to run new simulations until supply and demand are acceptably matched.

For this project the renewable energy is supplied by 85 Wp (Watt peak - the panel power output at STC) monocrystalline BP Solarex PV panels, each having an area of 0.66 square meters, and the backup system is composed of alkaline fuel cells. The excess energy from the PV panels is stored in the form of hydrogen. We can say that since the electrolyser has an efficiency of about 85%, and by comparing the electrical energy input to the electrolyser with the calorific value of the hydrogen output, 85% of the energy input to the electrolyser is realised in the form of the inherent energy in the stored hydrogen. Hydrogen is converted into electricity through the alkaline fuel cell with an efficiency of 70%, giving the total storage system an efficiency of 60%.

Hydrogen is stored under a pressure of 10 bar and has a calorific value of 35 kWh per cubic meter. Therefore the size of the hydrogen tank is determined from the total energy stored during period of excess energy production. The three demand profiles are based on an average electricity consumption of 400 kWh per month, which is the electricity consumption of a typical residential dwelling. 

The feasibility of the project has been studied under three different climates-

• Glasgow in Scotland

• Catania in Italy

• Phoenix in U.S.A

We studied the feasibility of the system in Phoenix because the location is representative of a desert climate. The system feasibility has been studied in Catania because of its Mediterranean climate. For both of these climates the demand profile used is based on the demand of an American dwelling using air conditioning. The demand profile for Scotland has been based on the electricity bills of a typical Scottish dwelling as air conditioning is not normally required in Scotland but more energy is required for heating during the winter.