Assessment of a Heat Pump-Based Wastewater Heat Recovery System for a Canadian University Campus

Abstract

Domestic hot water use in Canada accounts for roughly 19% and 8% of total residential and commercial building energy use, respectively. After use by building occupants, domestic hot water is disposed of in the sewer system where it mixes with other wastewater streams, typically reaching temperatures as high as 25 °C. Thus, an opportunity exists for recovering low-grade heat from wastewater and using it for meeting local thermal loads. This study uses the Carleton University campus, located in Ottawa, Canada, as a case study for wastewater heat recovery. The campus wastewater energy resource is estimated using a novel method, and a feasibility analysis is conducted on an industrial-scale heat pump that recovers waste heat from the university's main sewer outflow pipe. The recovered heat is used for heating water at the university's main athletics facility. Results show that installing a wastewater heat recovery system decreases greenhouse gas emissions and total annual costs by approximately 95% and 8%, respectively, relative to a base case natural gas heating system. Low-grade wastewater heat is an underutilized energy source that is both sustainable and accessible in most urban centres across Canada. Increasing the capacity of wastewater heat recovery systems has the potential to significantly increase energy savings and decrease greenhouse gas emissions in our energy system. Nomenclature Abbreviations GHG Greenhouse gases COP Coefficient of performance BC Base case scenario SHR Sewer heat recovery scenario BCRE Best-case recoverable energy estimation WCRE Worst-case recoverable energy estimation Variables _ V t ð Þ Building water consumption (m 3 /hr) _ V sewer t ð Þ Hourly variation of flow in the main sewer outflow pipe (m 3 /hr) V heated Volume of water heated by a building in a set time period (m 3) Q Energy used to heat hot water in buildings (kJ) q Density of water (kg/m 3) C p Heat capacity of water (kJ/kgK) T SP Temperature of the hot water tanks on campus (°C) T mains Temperature of fresh water in the water mains (°C) / Heating ratio V total Total volume of water used in a building during a set time period (m 3) T out Temperature of the outflow from a single building (°C) _ Q Sewer t ð Þ Energy in the main sewer outflow pipe (kW) T sewer t ð Þ Hourly variation of temperature in the main sewer outflow pipe (°C) _ Q recoverable t ð Þ Hourly variation of energy that is recoverable from the main sewer outflow pipe (kW) T discharge Temperature at which the effluent leaves the Carleton campus and enters the city sewers (°C) _ Q out;BC t ð Þ Energy output to meet the domestic hot water load in the base case scenario (kW) _ Q fuel t ð Þ Energy content of the fuel entering the boiler in the base case scenario (kW) g BC Average efficiency of the boiler in the base case scenario (%) _ W in;ep t ð Þ Energy required to pump effluent from the wet well to the heat pump evaporator (kW) _ Q evap t ð Þ Energy absorbed by the heat pump evaporator (kW) T LLT t ð Þ Temperature at which the effluent leaves the heat pump evaporator (°C) g Rate of acceleration due to gravity (m/s 2) DH Effluent pump system head loss (m) g ep Effluent pump efficiency (%) f Friction factor L Effluent pump system pipe length (m) d Effluent pump system pipe diameter (m) v Fluid velocity in the pipe (m/s) K Minor loss coefficient _ V max;ep Maximum volumetric flow rate through the effluent pump system (m 3 / s) v econ Economic velocity of water (m/s) _ Q out;SHR t ð Þ Energy output to meet the domestic hot water load in the sewer heat recovery scenario (kW) _ W in;HP t ð Þ Energy required to drive the heat pump compressor (kW) COP(t) Heat pump coefficient of performance (COP) AC Total annual cost ($/yr) NPC Net present cost ($) i Discount rate (%) C c Capital cost ($) M c Maintenance cost ($/yr) F c Variable fuel cost ($/yr) j Number of periods in net present cost calculation m CO 2 e GHG emissions in equivalent metric tonnes of CO 2 (tonnes) Assessment of a Heat Pump-Based Wastewater Heat Recovery … 147 AEO Annual energy output (MWh) e CO 2 e Greenhouse gas emission factor (kg/MWh) k Fuel escalation rate (%) n System lifetime (yr)