Final answer:
The main difference between demand defrost and timed defrost is that demand defrost is activated by sensors when needed, whereas timed defrost runs on a fixed schedule. Refrigerators, air conditioners, and heat pumps are most cost-effective when there's a small difference between the inside and outside temperatures, as this leads to a higher coefficient of performance (COP) and better efficiency.
Step-by-step explanation:
The difference between "demand defrost" and "timed defrost" relates to the operational control mechanism in refrigeration systems such as refrigerators, air conditioners, and heat pumps. Option a) correctly states that demand defrost is based on sensors while timed defrost operates on a fixed schedule. In demand defrost systems, sensors monitor frost accumulation on the evaporator coils, and when a certain threshold is reached, the defrost cycle is initiated. This is a more efficient method because the defrost only occurs when necessary. On the other hand, timed defrost systems initiate defrosting at regular intervals, regardless of the actual need, which can occasionally lead to unnecessary energy consumption.
Refrigerators, air conditioners, and heat pumps are most cost-effective when operating cycles have a small temperature difference between the inside temperature (T₁) and the outside temperature (T₀) because the coefficient of performance (COP) is higher when this temperature difference is smaller. The COP is a measure of efficiency for these systems, indicating the ratio of useful heating or cooling provided to energy consumed. A smaller temperature differential means that the system doesn't have to work as hard to transfer heat, leading to reduced energy usage and increased cost-effectiveness.