  Quantum thermodynamics supplies a consistent description of quantum heat
engines and refrigerators up to the level of a single few level system coupled
to the environment. Once the environment is split into three;a hot, cold and
work reservoirs a heat engine can operate. The device converts the positive
gain into power;where the gain is obtained from population inversion between
the components of the device. Reversing the operation transforms the device
into a quantum refrigerator. The quantum tricycle, a device connected by three
external leads to three heat reservoirs is used as a template for engines and
refrigerators. The equation of motion for the heat currents and power can be
derived from first principle. Only a global description of the coupling of the
device to the reservoirs is consistent with the first and second laws of
thermodynamics. Optimisation of the devices leads to a balanced set of
parameters where the couplings to the three reservoirs are of the same order
and the external driving field is in resonance. When analysing refrigerators
special attention is devoted to a dynamical version of the third law of
thermodynamics. Bounds on the rate of cooling when approaching the absolute
zero are obtained by optimising the cooling current. At low temperature all
refrigerators show universal behavior. Restrictions on the system imposed by
the dynamical version of the third law are studied.
