The pumps used in solar thermal installations come in both AC and DC models. It is important to choose a model that not only fits your electrical supply but which is also compatible with the rest of the system, in terms of construction materials.
Since the entire system carries water and the pump is electric, a galvanic reaction can occur which causes metals to corrode if you mix-and-match your pipes, heat exchanger and other equipment. This is especially true with cast-iron pumps. Although installing flanges to separate it from the piping can protect a pump from corrosion, it's often best to ensure that the pump casing is made of a compatible material (brass, stainless steel or bronze, for example).
AC pumps
Traditionally, AC pumps that are used in any heating system are also employed in solar thermal installations. They're normally 120-volt models that are mass-produced and so are available in a variety of configurations, styles and sizes. They're less expensive than specialized models, they're reliable and they have a long life. Some have multiple speed settings so you can optimize your system's flow rate.
When AC pumps break down, they're usually just replaced because they are low-cost and are not designed to be repaired. Some manufacturers do produce field-repairable models, called "cartridge circulators": these keep the volute (housing) separate from a removable cartridge inside. When the pump breaks down, the cartridge can be replaced.
DC pumps
If you're thinking of going one step further in your renewable energy setup and installing photovoltaic (PV) panels, you'll quickly learn that they provide DC electricity rather than AC. As a direct result of this, specialized solar thermal pumps are available which run off a direct current. They come in three types: brush, brushless and electronically driven – all three terms relate to the motor driving the pump.
The two traditional types are the brush and brushless pump. The first uses carbon brushes in contact with the commutator (a cylinder) in the motor. This physical contact means the brushes wear away over time and need to be replaced, unlike in brushless models which don't have brushes, obviously!
Brushless models use electronics instead of brushes and generally work better than brush models when they're powered by a PV panel. Brush pumps have trouble starting up under those circumstances and generally take longer to get going, though you can install a linear current booster (LCB) to give them an extra kick. Just remember to choose the right sized LCB for your pump and PV panel!
Modern innovation has provided the third type of pump. Electronically driven pumps don't use a conventional motor at all: they use electronics to spin the impeller. They're very efficient and several models have been designed especially for solar thermal installations, where flow rates are low to moderate. The more advanced models even come with extras like built-in maximum power point tracking (MPTT) to adjust the current coming from your PV panel to optimize power output.