Optimizing submersible pump regulation of a clogging prone groundwater heat pump system in Melhus, Norway

Abstract

The groundwater heat pump (GWHP) system at Lena terrasse in Melhus, Norway has suffered from clogging by iron oxide incrustations, sediment deposits, organic slimes and gas entrapment. This has necessitated annual rehabilitations of the injection well and heat exchanger, and questioned how the submersible pump should be regulated to maximize the seasonal performance factor (SPF = heat output/power input) and reduce the clogging risk. The submersible pump operates with variable speed drive to yield groundwater pumping rates (Q) between 4 and 16 l/s. The GWHP system has been continuously monitored by pressure, temperature, flowmeter and energy meter sensors for 2.5 years, and with semimonthly step-discharge tests. During three step-discharge tests hydrochemistry and suspended solids content were also measured. Monitoring data show that the monthly performance factor (SPFM) is negatively correlated with Q. At Q > 12-13 l/s, suspended solids concentrations increase. Heat exchanger clogging rates increase at Q < 6 l/s. Hydrochemistry is stable, regardless of pumping rate. All clogging rates appear to be independent of pumping rate in the range 6-12 l/s. Therefore, pumping rates are advised to be kept between 6-12 l/s, and minimized but adjusted to the heating demand to avoid freezing of the groundwater.