By Ajeet Singh
Significant amounts of energy are consumed in the heating of water for food production and other activities in buildings. The energy embodied in hot wastewater associated with kitchen drains of the hospitality and food services sector in the UK was estimated 1.24 TWh/yr. The capture of this energy has the potential to reduce greenhouse gas emission by 344 k tons Ce/yr [2]. If this energy could be recovered, it would directly attenuate the heating load requirement of the building, and hence the electricity bills. The EU has officially recognized wastewater as a renewable source of energy [3].
The present research focuses on recovery of heat from hot wastewater, flushed into the drainpipes of commercial kitchens. A grease trap is a system one can find in commercial kitchens for the removal of fat-oil-grease (FOG) from the kitchen wastewater. Grease traps present a convenient location where hot wastewater is temporarily retained, from where waste heat could be recovered, in comparison to sewer drains where hot wastewater quickly runs away. My objective is to recover embedded heat from the kitchens wastewater by retrofitting thermal recovery unit inside the grease trap (GT) device. The GT device integrated with thermal recovery unit is referred as hybrid GT system.
Fig. 1. Experimental prototype GT system integrated with thermal recovery unit: (a) Side view of GT system; (b) Thermocouple placement for temperature measurement at different locations; (c) Heat exchanger unit fitted to GT system;
The assessment of the hybrid GT system was conducted through:
the development of a laboratory prototype GT system at full scale and assessing its thermo-hydraulic performance (Fig. 1);
the development of a numerical model of the experimental prototype to gain deeper insights into its performance; and
the design of a second heat exchanger using the numerical model to improve the overall thermal recapture potential of the system (Fig.2).
A well devised methodology was followed in determining the performance of the hybrid GT system experimentally and computationally. The system integrated with the heat recovery unit was capable of recapturing a maximum heat of 1050 J per sec. The temperature of the clean water through the heat exchanging unit raised by 2-5 degrees Celsius and these results could facilitate preheating of cold water in buildings, lowering the primary heating requirement for conventional water heating devices.
About 20% of the heat was recaptured from the hot wastewater flowing through the GT system which earlier was going waste into the drainage system. At building level, about 21% of the hot water is consumed in kitchens [4]: mounting such a thermal recovery system could save around 20% of the energy from the high temperature kitchen wastewater. However, this research is in its development phase, and hence, it is very early to discuss its impact at building level.
Fig,2 Numerical model of the hybrid GT system
The preliminary results presented here were conducted using clean water to simulate the operation of the GT system, and future work is required to assess the performance of the system using wastewater containing FOG and to improve the design of the heat exchanger. Future work is also required to assess the impact of lowering wastewater temperature on FOG removal efficiency.
References
[1] J. R. Mihelcic and J. B. Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design, 2nd Edition. Wiley Global Education, 2014.
[2] Jan Spriet, Aonghus McNabola. Decentralized drain water heat recovery from commercial kitchens in the hospitality sector. Energy & Buildings 194 (2019) 247–259.
[3] DIRECTIVE (EU) 2018/2001 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 11 December 2018 on the promotion of the use of energy from renewable sources.
[4] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/48188/3147-measure-domestic-hot-water-consump.pdf