Types of Solar Water Heating (SWH) Systems

The type and complexity of a solar water heating system is mostly determined by:

• The changes in ambient temperature during the day-night cycle.
• Changes in ambient temperature and solar radiation between summer and winter.
• The temperature of the water required from the system.

The minimum efficiency of the system is determined by the amount or temperature of hot water required during winter (when the largest amount of hot water is often required). The maximum efficiency of the system is determined by the need to prevent the water in the system from becoming too hot (to boil, in an extreme case). There are two main categories of solar water heating systems. Passive systems rely on convection or heat pipes to circulate water or heating fluid in the system, while active systems use a pump. In addition, there are a number of other system characteristics that distinguish different designs:

• The type of collector used (see below)
• The location of the collector – roof mount, ground mount, wall mount
• The location of the storage tank in relation to the collector
• The method of heat transfer – open-loop or closed-loop (via heat exchanger)
• Photovoltaic thermal hybrid solar collectors can be designed to produce both hot water and electricity.

[edit] Passive systems

An integrated collector storage (ICS) system

A special type of passive system is the Integrated Collector Storage (ICS or Batch Heater) where the tank acts as both storage and solar collector. Batch heaters are basically thin rectilinear tanks with glass in front of it generally in or on house wall or roof. They are seldom pressurised and usually depend on gravity flow to deliver their water. They are simple, efficient and less costly than plate and tube collectors but are only suitable in moderate climates with good sunshine.

A step up from the ICS is the Convection Heat Storage unit (CHS or thermosiphon). These are often plate type or evacuated tube collectors with built-in insulated tanks. The unit uses convection (movement of hot water upward) to move the water from collector to tank. Neither pumps nor electricity are used to enforce circulation. It is more efficient than an ICS as the collector heats a small(er) amount of water that constantly rises back to the tank. It can be used in areas with less sunshine than the ICS. An CHS also known as a compact system or monobloc has a tank for the heated water and a solar collector mounted on the same chassis. Typically these systems will function by natural convection or heat pipes to transfer the heat energy from the collector to the tank.

Direct systems: (A) Passive CHS system with tank above collector. (B) Active system with pump and controller driven by a photovoltaic panel

Direct (‘open loop’) passive systems use water from the main household water supply to circulate between the collector and the storage tank. When the water in the collector becomes warm, convection causes it to rise and flow towards the water storage tank. They are often not suitable for cold climates since, at night, the water in the collector can freeze and damage the panels.

Indirect (‘closed loop’) passive systems use a non-toxic antifreeze heat transfer fluid (HTF) in the collector. When this fluid is heated, convection causes it to flow to the tank where a passive heat exchanger transfers the heat of the HCF to the water in the tank.

The attraction of passive solar water heating systems lies in their simplicity. There are no mechanical or electrical parts that can break or that require regular supervision or maintenance. Consequently the maintenance of a passive system is simple and cheap. The efficiency of a passive system is often somewhat lower than that of an active system and overheating is largely avoided by the inherent design of a passive system.

[edit] Active systems

Indirect active systems: (C) Indirect system with heat exchanger in tank; (D) Drainback system with drainback reservoir. In these schematics the controller and pump are driven by mains electricity

Active solar hot water systems employ a pump to circulate water or HTF between the collector and the storage tank. Like their passive counterparts, active solar water heating systems come as two types: direct active systems pump water directly to the collector and back to the storage tank, while indirect active systems pump transfer fluid (HTF), the heat of which is transferred to the water in the storage tank. Because the pump should only operate when the fluid in the collector is hotter than the water in the storage tank, a controller is required to turn the pump on and off. The use of an electronically controlled pump has several advantages:

• The storage tank can be situated lower than the collectors. In passive systems the storage tank must be located above the collector so that the thermosiphon effect can transport water or HCF from collector to tank. The use of a pump allows the storage tank to be located lower than the collector since the circulation of water or HCF is enforced by the pump. A pumped system allows the storage tank to be located out of sight.

• Because of the fact that active systems allow freedom in the location of the storage tank, the tank can be located where heat loss from the tank is reduced, e.g. inside the roof of a house. This increases the efficiency of the solar water heating system.

• New active solar water heating systems can make use of an existing warm water storage tanks (“geysers”), thus avoiding duplication of equipment.

• Reducing the risk of overheating. If no water from the solar hot water system is used (e.g. when water users are away), the water in the storage tank is likely to overheat. Several pump controllers avoid overheating by activating the pump at night. This pumps hot water or HTF from the storage tank through the collector (that is cold at night), thus cooling the water in the storage tank.

• Reducing the risk of freezing. For direct active systems in cold weather, the pump controller can pump hot water from the water storage tank through the collector in order to prevent the water in the collector from freezing, thus avoiding damage to the system

Active systems can tolerate higher water temperatures than would be the case in an equivalent passive system. Consequently active systems are often more efficient than passive systems but are more complex, more expensive, more difficult to install and rely on electricity to run the pump and controller.

[edit] Active systems with intelligent controllers

Modern active solar water systems have electronic controllers that permit a wide range of functionality such as full programmability; interaction with a backup electric or gas-driven water heater; measurement of the energy produced; sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display of error messages or alarms; remote display panels; and remote or local datalogging.

A typical programmable differential controller

The most popular pump controller is a differential controller that senses temperature differences between water leaving the solar collector and the water in the storage tank near the heat exchanger. In a typical configuration, the controller turns the pump on when the water in the collector is about 8-10°C warmer than the water in the tank and it turns the pump off when the temperature difference approached 0 °C. This ensures the water always gains heat from the collector when the pump operates and prevents the pump from cycling on and off too often.

Although the pumps of most active systems are driven by mains electricity, many active solar systems obtain energy to power the pump by a photovoltaic (PV) panel. The PV panel converts sunlight into electricity, which in turn drives the direct current (DC) pump. In this way, water flows through the collector only when the sun is shining. The DC-pump and PV panel must be suitably matched to ensure proper performance. The pump starts when there is sufficient solar radiation available to heat the solar collector. It shuts off later in the day when the available solar energy diminishes. Several DC-pumps are intelligent and employ maximum power point (MPP) tracking to optimise pump rate, for instance during periods of small amounts of electricity from the PV panel during cloudy weather. The controller is sometimes used to prevent the pump from running when there is sunlight to power the pump but the collector is still cooler than the water in storage. The main advantage of a PV-driven pump is that hot water is always available during a power outage. The pump is operated by the sun and is completely independent from mains electricity. Some differential controllers use power from the PV panel when sunlight is available, but use mains electricity when light is not available.

[edit] Active systems with drainback

A drain-back system is an indirect active system where heat transfer fluid circulates through the collector, being driven by a pump. However the collector piping is not pressurised and includes an open drainback reservoir. If the pump is switched off, all the heat transfer fluid drains into the drainback reservoir and none remains in the collector. Consequently the collector cannot be damaged by freezing or overheating.^[15] This makes this type of system well-suited to colder climates.

[edit] Freeze protection

Freeze protection measures prevent damage to the system due to the expansion of freezing transfer fluid. Some systems drain the transfer fluid from the system when the pump stops. In indirect systems (where the transfer fluid is separated from the heated water), this is called drainback and in direct systems (where the heated water is used as the transfer fluid) it is called draindown. Many indirect systems use anti-freeze (e.g. glycol) in the heat transfer fluid. This approach is simpler and more reliable than drainback and is common in climates where freezing temperatures occur often.

In both direct and indirect systems, automatic recirculation may be used for freeze protection. When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the tank.

In some direct systems, the collectors are manually drained when freezing is expected. This approach is common in climates where freezing temperatures do not occur often.

[edit] Overheat protection

The water from the collector can reach very high temperatures in good sunshine, or if the pump fails. Designs should allow for relief of pressure and excess heat through a heat dump. Almost all systems have pressure relief valves through which excessive water pressure or steam can be vented. Active systems often cool the water in the storage tank by circulating hot water through the collector at night (when solar energy does not heat the collector).

[edit] A rough comparison of solar hot water systems