Solar Water Heating

Introduction

The sun can be harvested for heating up water. This works especially well anytime the sun is out. Even in cloud cover, the ambient temperature can help raise water temperature to reduce reliance on mechanical systems. All solar water heating systems should be paired with a mechanical system for use off-sun times and when extra heat is needed beyond the capacity of the solar heating system.

Types of Solar Water Heating

• Batch System: Heats water directly in a black painted tank inside a glazed box. Subject to nighttime heat loss and freezing.
• Thermosiphon: Relies on natural movement of heated water to recirculate itself in a passive loop.
  • System must be simple and designed to reduce friction in the pipe.
• Closed-Loop Active System: Most common. A separate non-freezing liquid is circulated by pumps through the solar collectors into a heat exchanger where the water is heated. A differential controller senses when the collector is lower than the stored water and will turn the pump off.
  • This system suffers some efficiency loss due to its need for a heat exchanger, and costs more to operate the pumps and exchanger.
• Drain-Down System: A direct, active system that solves the problem of freezing by automatically draining the water from the collectors when outside is near freezing.
  • Water is wasted every time this occurs, so this system should only be used where freezing is infrequent (mild climates).
• Drain-Back: Indirect, active system that uses water as the heat-collected fuel. The heated water is pumped into the heat exchanger where hot water is heated. When the controller senses freezing temperatures, the system is shut down and the water drains back into the solar storage tank.
• Phase Change System: Uses a Phases Change Material (PCM) to store heat (sensible and latent) through its phase state change throughout the day. At night, it may change states again, releasing the heat it gained throughout the day.
  • There are ongoing advancements to this system.