To be more comprehensible, we could replace the attribute names of SolarThermalSystem and PhotovoltaicThermalSystem:
eta0 -> zero heat loss efficiency (scaletype[0.1]), also called optical efficiency
a1 -> linear heat loss coefficient (numerical)
a2 -> quadratic heat loss coefficient (numerical)
To be more comprehensible, we could replace the attribute names of SolarThermalSystem and PhotovoltaicThermalSystem: eta0 -> zero heat loss efficiency (scaletype[0.1]), also called optical efficiency a1 -> linear heat loss coefficient (numerical) a2 -> quadratic heat loss coefficient (numerical)