Diesel Exhaust Waste Heat Recovery for Domestic Hot Water: A Parametric and Economic Analysis

Abstract This study conducts a detailed thermal and economic analysis of a heat recovery concentric tube heat exchanger (HR-CTHE) designed to utilize diesel engine exhaust waste heat for domestic water heating. Using a counter-flow configuration, we systematically vary hot exhaust inlet temperatures (500–900 K), Reynolds numbers on both hot and cold sides (1,000–20,000), and geometric parameters (inner tube diameters from 0.03 to 0.15 m and diameter ratios of 1.25–3) to assess their impact on system performance. The results indicate that higher exhaust inlet temperatures and increased hot-side turbulence (e.g., Re h = 20,000) dramatically enhance the heat recovery rate, surpassing 25,000 W, and raise the preheated water temperature above 310 K under optimal conditions. From an economic perspective, these thermal improvements translate into significantly shortened discounted payback periods, reaching as low as a few months. Furthermore, the baseline energy source—electricity or natural gas—profoundly influences the payback period, with systems displacing more expensive electricity achieving notably quicker returns. These findings highlight the HR-CTHE’s potential as a cost-effective and efficient option for integrating waste heat recovery into residential hot water systems.