•This illustrates that the quality of energy is an important factor in determining the efficiencies of systems.  E.g. for the same amount (quantity) of total energy, it is easier – more efficient – to produce work from a high temperature reservoir than a low temperature reservoir. Consequently, extracting energy from low-temperature reservoirs such as rivers and lakes is not very efficient.  E.g. solar pond/lake have typical efficiencies of around 5% •Also, work is in general more valuable – of a higher quality - relative to heat, since work can convert to heat almost with almost 100% efficiency but not the other way around.  Energy becomes less useful when it is transferred to and stored in a low-temperature reservoir.