Quick Answer
To calculate the collector area for a drainback system, determine the required water flow rate, then use the collector area equation, which typically involves the desired flow rate, system temperature difference, and collector efficiency.
Calculating Collector Area Requirements
When sizing a drainback solar hot water system, the first step is to determine the required water flow rate. This can be calculated using the Gifford-McMillan equation, which considers the volume of water needed to maintain a comfortable water temperature during periods of low solar radiation. A typical flow rate calculation for a drainback system might be 2-4 gallons per minute (GPM) per person, so for a family of four, the required flow rate would be 8-16 GPM.
Collector Area Equation
The collector area can be calculated using the following equation: Collector Area (ft^2) = (Flow Rate (GPM) x 60) / (ΔT x η). Here, ΔT is the temperature difference between the system outlet and the desired water temperature (usually around 30-40°F), and η is the collector efficiency (typically 0.7-0.8 for a drainback system). For example, if the desired flow rate is 10 GPM and the temperature difference is 35°F, the collector area would be approximately 17.1 ft^2 (using an efficiency of 0.75).
System Efficiency and Component Selection
When selecting components for a drainback system, it’s essential to consider the system efficiency and the performance characteristics of the collector. A well-designed system with a high efficiency collector and proper insulation can achieve a system efficiency of 70-80%. In addition to the collector area, the system designer should also consider the pump size, piping material, and heat exchanger selection to ensure efficient and reliable operation.
Find more answers
Browse the full Q&A library by topic, or jump back to the topic this question belongs to.