Energy for off-grid homes: reduced cost through hybrid system and energy efficiency optimisation

Abstract

Hybrid systems, which combine two or more types of energy technologies, including renewable energy, are often the most cost effective way to provide electricity to off-grid houses; houses not connected to the electrical grid. Hybrid systems involve a significant initial expenditure due to the high costs of the equipment required. To find the optimal design of hybrid system that will meet the energy demand at the lowest cost, the use of optimisation software tools has become standard practice. Hybrid system costs can be reduced further by applying energy efficiency measures (EEM) to decrease energy demand. Examples of these measures include adding insulation to the building envelope, or replacing old appliances with more efficient ones. The application of EEM followed by the optimisation of a hybrid system is recommended for achieving the lowest cost system. Although EEM lower the energy demand and cost of hybrid systems, their application involves costs that should be considered when analysing the total cost of the system. Given that many possible combinations of these measures can be applied, it is essential to find the right combination of EEM and hybrid system technologies that will result in the lowest cost in order to determine which of these combinations should be considered to minimise the total cost. Methods used to find the right combination of EEM and hybrid systems are reviewed in this research, but no method was found that could calculate the most cost effective combination for an off-grid house. Methods for finding the lowest cost design were found for grid-tied and low energy houses, which used optimisation analysis that considered both EEM and energy supply systems; however, these methods cannot be applied for off-grid houses because they do not consider all the variables available to an off-grid house, such as the full range of hybrid system technologies. The hypothesis of this research proposes a new process for finding the lowest cost combination of EEM and hybrid systems. The hypothesis was tested using a NZ-based hypothetical case. The new process consists of a loop that combines a building energy and EEM optimisation tool (BEopt) and a hybrid system optimisation tool (HOMER). The optimisation of both EEM and hybrid systems determines the total cost of a specific combination of EEM and hybrid system configuration. This new process was compared with both the Base Case and the standard process, which was the application of any EEM, followed by the optimisation of the hybrid system. The results showed that the new process found the most optimum solution, which was 10% cheaper than the Base Case and 5% cheaper than the standard process.