Heat pump extraction

The heat pump extraction as a function of air temperature. Above 11 °C the heat extraction rate equals the base extraction rate, s_h1. Below this temperature the heat extraction increases to a maximum rate below the threshold temperature, T_hpmax. s_h2 = 100 000 Jm^-2/day/°C (blue line), s_h2 = 150 000 Jm^-2/day/°C (green line).