An efficient thermal water pump for electricity-free recovery of industrial waste heat

Abstract

Industrial waste heat (IWH) below 200 ℃ forms an immense source of free and clean energy, which contributes directly to global warming if not recovered. This paper presents a novel use of thermal water pump (TWP) to efficiently recover low-grade IWH without using electricity. TWPs use phase-change of working fluid for discharge and suction of pumped fluid. Existing TWPs have low pumping efficiency and discontinuous discharge, which is uneconomical and impractical for industrial processes. This paper optimizes TWP design for industrial purposes: first, by identifying parameters controlling TWP pumping performance and optimizing system design. Particularly, vapor flow path is optimized to increase efficiencies by 10 folds. Additionally, heat rejected from the condenser is recycled to heat discharge water. The energy recovery efficiency (ERP) is achieved comparable to forced and natural circulation solar water heaters (30–60 %). Second, continuous discharge is produced by suppressing fluctuation by 80 % using an inline accumulator. The highest ERP of 39 % is achieved for 16 L/h discharge at 48 ℃ over 70 ℃ temperature difference. A payback period of 4.2 years is estimated for 5 kW system. Our TWP can enable an annual recovery of 4087 kWh equivalent to $940 USD. A levelized energy cost of 0.23 USD/kWh is achieved equivalent to photovoltaic (PV) system (0.21–0.26 USD/kWh). Our TWP is intended to supply hot saline to our patented desalination system without electric pumps. This study highlights the potential of TWPs to efficiently and economically recover the untapped low-grade IWH, and paves path for a sustainable future.