Final answer:
Model drain water heat recovery systems separately from other systems, in accordance with existing guidelines. This approach is integral to analyzing energy and water systems efficiently, independent of whether they use renewable energy sources.
Step-by-step explanation:
The student's question pertains to the modeling of drain water heat recovery systems within a broader analysis likely of an energy or water system. The correct approach would be to model them separately from other systems, following established guidelines. Modeling such systems is not limited to when they use renewable energy sources, like solar, wind, or hydroelectric, but it's important due to their potential in energy savings and efficiency in various setups, including geothermal and nuclear power plants where water systems must remain separate to prevent contamination. In the context of energy systems that utilize renewable sources, drain water heat recovery can be particularly beneficial. For example, in a solar electric generating system, waste heat recovery could improve efficiency, as demonstrated by the systems that use parabolic troughs to concentrate sunlight for energy generation. When considering how to add energy to or remove energy from a system, transferring energy by heat or by doing work are the methods used. Both approaches are relevant for detailed modeling of energy systems and understanding how drain water heat recovery can affect the system's overall performance.