Apparatus and method for generating electricity. The apparatus includes one or more first components configured to extract heat from at least a first fluid flow at a first temperature to one or more devices configured to convert thermal energy to electric energy. The first fluid flow is in a first direction. Additionally, the apparatus includes one or more second components configured to transfer heat from the one or more devices to at least a second fluid flow at a second temperature. The second temperature is lower than the first temperature, and the second fluid flow is in a second direction. Each first part of the first fluid flow corresponds to a first shortest distance to the one or more devices, and the first shortest distance is less than half the square root of the total free flow area for a corresponding first cross-section of the first fluid flow.