2009 Volume 31 Issue 1 Pages 39-50
There are quite a few hot springs and spas in Japan, where people utilize hot spring water directly for bathing and waste it. Waste heat from baths in those spas could be reused for the other purposes. Cascade systems for utilizing waste heat for room heating and for hot water supply are studied in this paper, focusing on the economical and the environmental benefits.
In this paper, a typical hot spring bath in Tohoku Area which has 60 [L/min] and 40 [°C] of hot spring waste water is assumed. We discuss cascade systems where the heat demand is the only room heating of bathroom and hot water supply to showers.
A simulator with MATLAB Simulink was developed to analyze dynamic behavior of four options of the cascade systems; case 1 is a system in which waste heat is supplied to a heat pump first for room heating, and to a heat exchanger second for hot water supply to a water heater; case 2 is a system in which the order of the heat pump and the heat exchanger is reverse to the case 1; in case 3, waste heat is utilized only for the hot water supply through the heat exchanger; in case 4, waste heat is used only for the room heating by the heat pump system. A conventional fossil fuel system that does not utilize waste heat was also analyzed as case 5 for a reference case.
The estimated heat demand for the hot water supply to showers is 6-15 times larger than that for the room heating. Hence, nearly 2/3 of the running costs and CO2 emission in the case 4 and 5 are cut in the cases 1-3 which has the heat exchange system to supply hot water. The payback time for the initial costs of the case 3 is the shortest and it is only 1-2 years. The utilization of waste heat for the demand for hot water supply to showers is generally the most efficient way to improve the economical and environmental benefits.
