Solar Water Heating--Revised & Expanded Edition. Bob Ramlow
4.4: Plate heat exchanger
Heat exchangers can be constructed using either pipes or plates. Two fluids are passed next to each other, separated by a membrane. Heat transfers across the membrane from one fluid to the other. Always plumb heat exchangers in a counter-flow arrangement, as this is the most efficient way to transfer heat. This means that the two fluids will need to flow in opposite directions when passing through the heat exchanger. This allows for the greatest difference in temperature between the two fluids, which increases the rate of heat transfer.
A plate heat exchanger consists of a number of plates spaced apart and capped around the sides. Separate waterways are designed into them to allow the different fluids to pass through adjoining spaces in a zig-zagging counter-flow pattern. This back-and-forth arrangement means that plate heat exchangers always require a pump on both waterways, as they will not convect. This type of exchanger can pack a large amount of surface area into a small package. Because heat transfer is directly related to the amount of surface areas that are available, you are unlikely to have too much heat transfer surface area. Because they have so much surface area in such a small package, plate heat exchangers also have very small waterways. Consequently, in hard-water conditions, scale can easily build up, causing reduced heat transfer or clogging. When installing one of these, you may want to place boiler drains and isolation valves on the domestic water side pipes to provide the means to periodically flush out the heat exchanger. Remember to always use a nontoxic solution when doing so. We prefer a product that is enzyme-based, making it biodegradable, non-corrosive and safe for septic systems.
A tube-in-shell heat exchanger is essentially a smaller pipe (or pipes) inside a larger pipe or tube. One fluid is circulated through the inner pipe(s) and the other fluid circulates through the outer pipe. The material of the inner tube separates the fluids. Tube-in-shell heat exchangers come in either single-wall or double-wall versions. These heat exchangers can be configured in straight lengths or coiled. The coiled configuration always requires a pump like the plate exchangers, but the straight ones can thermosiphon on the water side if properly designed for that purpose.
Pumps
Pump Construction
It is important that all the components within a solar loop are made of compatible materials. When different materials are mixed in a plumbing circuit, galvanic reactions can take place that speed up oxidation and deterioration of the components. We prefer to use brass, stainless steel or bronze pumps in the solar water heating system, both in the solar loop and for all potable water plumbing. Some installers choose to use a cast-iron pump on the solar loop because it is often significantly less expensive. Though this may lead to corrosion, if the pump is separated from the piping loop through pump flanges, the gaskets often provide enough separation to prolong their use. The impellers within the pump are almost always made of stainless steel or plastic, so they are not a problem. It is the pump housing that is critical.
Figure 4.5: Tube-in-shell heat exchanger
AC Pumps
The traditional type of pump used in a solar water heating system is a 120-volt AC pump. These pumps are readily available in a variety of sizes and are also used in traditional hydronic heating systems. Because these pumps are mass produced, they are less expensive than specialty pumps designed for the solar industry. There are many manufacturers of these pumps, which come in a variety of configurations. These pumps are reliable and last for a long time. Because of their low price, many of these pumps are not field repairable, so when they break, they are just replaced. A few manufacturers make pumps called cartridge circulators. These have a cartridge inside the pump that can be replaced without having to take the whole pump housing (called the volute) out of the system when doing a repair. Some AC pumps have multiple speed settings so that the flow rate can be optimized for the system.
Figure 4.6: AC pump
DC Pumps
Many solar water heating systems are installed with a DC pump so that it can be powered directly by a solar electric or photovoltaic (PV) module. DC pumps come in three configurations: brush, brushless and electronically driven. This designation relates to the type of motor that drives the pump.
Brush-type motors use brushes made of carbon that contact the commutator, which is a cylinder in the motor. These brushes wear away over time and need to be replaced periodically. Brushless motors are electronically commutated and, obviously, have no brushes. Brushless motors work better in photovoltaic direct applications, but brush-type motors can work fine. Brush-type motors have a harder time starting up when powered directly from a PV panel. They will start, but not as quickly as brushless ones.
A linear current booster (LCB) can be installed between the PV collector and the brush-type motor to help the brush-type motor start more easily. LCBs need to be properly sized to match the PV collector and the pump, so do your homework if using an LCB.
Electronically driven DC pumps fall into the category of brushless pump, but they don’t have a conventional motor. These pumps use electronics to spin the impeller. Several brands of this type of pump have been developed specifically for the solar water heating industry, where low to moderate flows are required. One model even has built-in circuitry with maximum power point tracking (MPPT), which adjusts the voltage and current coming from the PV module to maximize the amount of power it is producing. Neat stuff.
Piping and Pipe Insulation Piping
The piping in a solar loop is subjected to a wide range of temperatures, varying from 300°F to–30°F (or lower). Copper tubing is the best kind of pipe to use for the solar loop. It can withstand this temperature range and is very durable and easy to install. Copper tubing comes in various grades and is classified by wall thickness and rigidity. Soft copper (annealed) can be bent; hard copper (drawn) is very rigid. The heavier the wall thickness, the more rigid it is. Type M copper tube is thin walled; type L is medium walled; and type K is heavy-walled. We suggest using type L for the solar-loop piping. Use type K for underground piping runs. We suggest using hard copper in all instances except underground piping runs. All types of copper pipe have the same exterior dimensions, so all fittings are made the same and come in one standard size for use with all types. However, the exte-rior diameter is actually ⅛” larger than its nominal designation.
Figure 4.7: DC Pump
It is important to use only copper pipe, or in some cases stainless steel, for the hot supply pipe in the solar loop. People often want to use a flexible product such as Pex tubing or rubber hose. These products will not last very long and will deteriorate well before the system wears out. In his repair business, Bob had to replace a lot of hoses that were used in the 1980s. Not a single installation from those days that used rubber hose is still operating in Wisconsin (to our knowledge). Pex tubing will also fail in a very short time. We know of several attempts to use Pex tubing for the hot supply line, and the failures were always within the first year. It is possible to use Pex tubing for the return line back to the collectors, but it is important to terminate the Pex at least 10 feet from the collectors. The Pex must never be used for an outside pipe run unless it is buried. We suggest that you consider Pex only for use in the solar loop when burying the return line out to a ground-mounted array.
There is one alternative to copper for use in the solar loop. In the past few years several solar companies have developed a corrugated stainless steel piping that is both flexible and durable enough to handle the high temperatures of a solar system. Its flexibility simplifies installation