There are different types of solar water heaters; this page covers the drainback system, a tested and reliable choice for cold climates and compares it with PV electric solar water heating.
Typical drainback water heater systems comprise:
1) solar panels;
2) a controller;
4) a storage tank (say, 80 gallon);
5) a small drainback reservoir tank (say, 10 gallon/38 liters);
6) a heat exchanger (usually internal to one of the tanks) and
7) sensors and other minor accessories.
Modern drainback systems use tube panels and distilled water or a glycol antifreezing mixture as the heating fluid.
How drainback systems works
When the sensors attached to the solar collectors detect useful sunlight available - which happens when the temperature in the solar panels is higher than the temperature of the water at the bottom of the storage tank - the system begins to work.
The controller will start the pump (or pumps) and will route a heating fluild (typically distilled water) from the reservoir tank to the solar panel and back, in a continuous loop.
As the distilled water passes through the solar collector, it heats up, and passes its heat to the potable water – usually through a heat-exchanger connected to the tank. Pumps will continue circulating the water through the system as long the controller detects useful solar heat.
Design and storage tank location
The drainback solar design rests largely on the gravity-fed principle (the system is technically called unpressurized, due to it). The draining of the fluid into the reservoir follows the gravity-fed principle and requires the collector to be located higher than the storage tank.
Advantages: reliability and Performance
Drainback systems are reliable and require less maintenance than other cold-climate solar thermal systems. The heating fluid (distilled water) rarely has to be changed.
The SRCC (Solar Rating and Certification Corporation: a certification entity that sets solar equipment standards for the solar industry), recommends them for cold climates due to their relative simplicity and lifespan.
They have less moving components (valves, air vents…) than pressurized antifreezing systems, and are less prone to pump fails and other damages - though they require regular maintenance.
Disadvantages of drainback solar systems: PV Solar Electricing water heating can be a better choice
The disadvantages of drainback solar systems are those of solar thermal hot water systems for cold climates: high prices, significant maintenance requirements and the need of a backup system.
They rarely are a cost effective choice. See: Solar water heating paybacks.
Hence the growing appeal of solar photovoltaic systems (PV solar electricity).
Current solar photovoltaic (PV) systems are a lot cheaper than some years ago, and the electricity they produce can be used to heat water...
A dedicated 1 kW photovoltaic system for water heating costs less than a drainback system, though prices vary a lot (a 1 kW may cost $4.000 in the US and Canada, less than a typical drainback system).
Anyway, there is a way of lowering the cost: installing a large PV system for multiple residential uses, instead of a dedicated system. In such a context, increasing the capacity of a photovoltaic system by 1 kW - from 4 kW to 5 kW, for instance - to also respond to hot water heating needs, may cost you $2,000 or less.
Besides, PV systems can be connected to cheap electric-resistance water heaters or to a heat pump water heater, without the need of backup system. It's a question of design.
Obvisouly, there are prerequisites that should be met, especially good solar resources.