Conventional natural gas-fired boilers exhaust flue gas direct to the atmosphere at 150–200 °C, which, at such temperatures, contains large amount of energy and results in relatively low thermal efficiency rang- ing from 70% to 80%. Although condensing boilers for recovering the heat in the flue gas have been devel- oped over the past 40 years, their present market share is still less than 25%. The major reason for this relatively slow acceptance is the limited improvement in the thermal efficiency of condensing boilers. In the condensing boiler, the temperature of the hot water return at the range of 50–60 °C, which is used to cool the flue gas, is very close to the dew point of the water vapor in the flue gas. Therefore, the latent heat, the majority of the waste heat in the flue gas, which is contained in the water vapor, cannot be recovered. This paper presents a new approach to improve boiler thermal efficiency by integrating absorption heat pumps with natural gas boilers for waste heat recovery (HRAHP). Three configurations of HRAHPs are introduced and discussed. The three configurations are modeled in detail to illustrate the significant thermal efficiency improvement they attain. Further, for conceptual proof and validation, an existing hot water-driven absorption chiller is operated as a heat pump at operating conditions similar to one of the devised configurations. An overall system performance and economic analysis are provided for decision-making and as evidence of the potential benefits. These three configurations of HRAHP pro- vide a pathway to achieving realistic high-efficiency natural gas boilers for applications with process fluid return temperatures higher than or close to the dew point of the water vapor in the flue gas.
As part of the second phase of the OptiEnR research project, the present work focuses on optimizing multi-energy district boilers by adding thermal storage tanks to the plants. First, both a parametric study and a simulation-based evaluation of the thermal losses are carried out in order to design the hot water tanks. Next, a sequential management approach, based on the power demand and the characteristics of the biomass unit(s), is defined with the aim of improving efficiency. Energy and economic criteria are proposed and evaluated in order to highlight the configurations that meet needs and expectations. The way thermal energy storage impacts on the boiler units dynamics is evaluated in simulation. Finally, the proposed approach has been applied to two multi-energy district boilers equipped with one and two biomass units, respectively. The plants are managed by Cofely GDF-Suez, our industrial partner in the project. The results highlight the ability of a hot water tank (when it is optimally designed and managed) to improve the operation of a multi-energy district boiler and realize significant economic savings. As a key point, the excess of energy produced by the biomass unit(s) during low-demand periods can be stored and released when demand is high, instead of engaging a gas boiler.