Integration of gas instantaneous auxiliaries with renewable energy residential water heaters

Abstract

Gas instantaneous water heaters are increasingly used as auxiliary energy sources for renewable energy domestic hot water systems. Currently the gas used by these systems is measured and modelled only very approximately. Accurately assessing consumption is important as gas can supply up to 50% of the energy. Gas auxiliaries used in these arrangements are also predominantly designed as stand-alone devices and typically integrated into renewable systems in a very basic manner. This research seeks to fill two important gaps. First, how does the gas instantaneous water heater actually perform as part of a renewable water heater? Second, once performance can be measured and modelled accurately, what improvements in energy use might be made through novel ways of integrating the gas auxiliary with the renewable energy water heater? This study adopts a broad approach in examining these gaps. A number of existing test methods for gas instantaneous, air source heat pump, solar and electric storage water heaters are reviewed in detail. A number of different gas instantaneous water heater test standards are compared experimentally. The effect of water use patterns on the energy use of various water heating technologies is investigated both experimentally and with modelling. The carbon emissions of a number of different types of water heater is calculated after a detailed review of CO2 emissions of the New Zealand electricity generating system during both dry and wet years. A new experimental test method is developed to characterise the steady state performance of a gas instantaneous water heater. This performance characterisation is then used in a TRNSYS model together with established models of renewable energy water heaters to predict energy consumption. Integrated systems using novel control and hydraulic connection arrangements are then compared to other water heating systems. The current methods of determining the gas consumption of auxiliary water heaters may understate energy use by as much as 15%. Improved control system integration saves an average of 17% in energy and 11% in carbon dioxide emissions compared to other high efficiency water heating systems. Improved hydraulic arrangements result in 4.5% savings. The change in energy conversion ratio with differing water use patterns of some water heaters can exceed the differences exhibited between technology types and competing models of similar technology. If consumers are to choose their water heaters based on comparable running costs or emissions, accurate measurement and modelling of gas auxiliaries is required. Current test methods do not appear to provide the required accuracy. There are also opportunities for significant improvement in energy use through better integration of gas auxiliaries with renewable energy residential water heating systems.