{"id":103,"date":"2022-05-10T18:07:13","date_gmt":"2022-05-10T22:07:13","guid":{"rendered":"https:\/\/opentextbc.ca\/plumbing3g\/chapter\/types-of-systems\/"},"modified":"2022-10-03T16:28:01","modified_gmt":"2022-10-03T20:28:01","slug":"types-of-systems","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/plumbing3g\/chapter\/types-of-systems\/","title":{"raw":"Learning Task 1","rendered":"Learning Task 1"},"content":{"raw":"Any device or combination of components intended to convert solar radiation into useable heat can be classified as a solar thermal system<\/em>, and within that designation lie the two classifications of systems, which are passive <\/em>and active<\/em> systems.\r\n

Passive Solar Thermal Systems<\/h1>\r\nPassive systems are solar thermal systems that collect and distribute heat without the use of fans or circulators. A building with large south-facing windows and concrete floors can absorb heat during the day and release it to the room in the evening. An example of a \u201cdirect gain\u201d passive solar design is seen in Figure 1.\r\n\r\n[caption id=\"attachment_102\" align=\"aligncenter\" width=\"660\"]\"\" Figure 1 Passive building heating[\/caption]\r\n\r\nAn appropriate amount of eave overhang allows the rays from the low winter sun into the building while blocking the more overhead rays in the summer. In this way, heat gain is maximized in the winter and minimized in the summer.\r\n\r\nPassive solar systems can also be used to heat domestic hot water, as seen in Figure 2.\r\n\r\n[caption id=\"attachment_102\" align=\"aligncenter\" width=\"700\"]\"\" Figure 2 Passive solar DHW system[\/caption]\r\n\r\nA sloped solar collector is mounted below an insulated storage tank. When the sun\u2019s rays warm the water in the collector, it expands in volume, becomes less dense and is pushed upward by gentle convection currents created by the cooler water below it into the insulated tank. This is known as \u201cthermosiphoning\u201d. As long as there is more heat in the collector than in the storage tank, this process will continue. Once the sun sets, or at any time where the temperature in the collector falls below that of the tank, the flow stops. These systems work well in warmer climates where the possibility of freezing is minimal or non-existent, but in colder climates, the collector and any parts of the system that may freeze must be drained. Figure 4 shows the insulated storage tank as a pre-heater for the electric water heater. This can reduce the energy costs caused by the electric elements. If water temperatures higher than 140\u00b0F (60\u00b0C) are expected, a thermostatic mixing valve must be installed to avoid scalding water from reaching any plumbing fixtures. High temperatures are possible in any type of solar water system, especially in times of prolonged sunny weather, especially if hot water demand is low.\r\n

Active Solar Thermal Systems<\/h1>\r\nAny system that uses a circulator to move a fluid through a solar collector is known as an active solar thermal system. The use of a circulator (pump) to move water through the collector makes the design considerations of these systems almost limitless. We will focus our study of solar thermal systems on those using water or liquid as the heating medium, and we will start by looking at the heart of the system, the collector.\r\n

Solar Collectors<\/h2>\r\nThere are two main types of solar collectors used in domestic hot water and space heating; flat plate type and evacuated tube type.\r\n

Flat Plate Collectors<\/h2>\r\nFlat plate type collectors have been on the market for many decades and can also be home-built. They are essentially many small-diameter copper tubes that are bonded to a copper sheet and run vertically within the housing between an upper and lower header, as shown in Figure 3.\r\n\r\n[caption id=\"attachment_102\" align=\"aligncenter\" width=\"855\"]\"\" Figure 3 Flat plate collector[\/caption]\r\n\r\nThe copper sheet absorbs the sun\u2019s heat and transfers it to the copper tubes. The thermosiphon principle moves the heated water to the upper header where it is pumped out to the system. The heat-absorbing feature of the copper sheet is augmented by painting or electro-plating with a matte black finish. The assembly of headers, tubes and sheet are contained within an aluminum frame having a high-impact tempered glass cover and high temperature insulation between the back of the copper sheet and the back of the frame. All materials have to be capable of withstanding temperatures in excess of 350\u00b0F (177\u00b0C) on days where there is minimal or no water flow through the collector. As well, the collector must withstand the impacts from hailstones and other objects. Flat plate collectors have been used for many decades and, due to their simple design, are capable of being fabricated on-site.\r\n

Evacuated Tube Collectors<\/h2>\r\nEvacuated tube collectors are a series of glass tubes inserted into a special header. Each tube consists of an inner and an outer glass shell, with the atmosphere removed from the space between them, which makes it operate similarly to a Thermos\u00ae bottle. Inside the inner glass tube is a sealed copper tube that has a special liquid inside it. When exposed to sunlight, the liquid in the copper tube vapourizes and rises to the top of the tube, where the heat of the vapour is transferred via conduction through a copper \u201ccap\u201d to a sealed socket in the liquid-filled header. The header\u2019s liquid, which can be either water or a water-glycol mixture, is then moved through the system by pumps. There is only one header at the top of the tubes, with inlet and outlet at opposite ends.\r\n\r\n[caption id=\"attachment_102\" align=\"aligncenter\" width=\"528\"]\"\" Figure 4 Evacuated tube collector in position[\/caption]\r\n\r\n[caption id=\"attachment_102\" align=\"aligncenter\" width=\"763\"]\"\" Figure 5 Evacuated tube collector[\/caption]\r\n\r\nIt is important to note that the liquid in the tubes never mixes with the system liquid within the header. When the vapour in the tube cools, it condenses and falls back to the bottom of the copper tube, ready to repeat the cycle. Headers can be \u201cganged\u201d together for extra capacity, and piped similarly to heat transfer units in a hydronic heating system. If a tube is damaged, it is simply pulled from the header and replaced; the \u201csocket\u201d that the tube is inserted into is sealed to the header.\r\n

Swimming Pool Solar Collectors<\/h2>\r\nOne of the most common residential uses of solar energy is in the heating of swimming pool water. These collectors are normally of the uninsulated, unglazed flat plate variety, made of UV-stabilized polymers that are compatible with the pool water\u2019s chemistry. A sheet of small-diameter plastic tubes is connected to an upper and a lower header and is laid on a flat surface such as a roof, with no insulating material between them. Their design is similar to that of a flat plate collector, with the exception that there is no enclosure around the headers or tube sheet.\r\n\r\n[caption id=\"attachment_102\" align=\"aligncenter\" width=\"518\"]\"\" Figure 6 Swimming pool collector[\/caption]\r\n\r\nIdeally, the roof would be angled appropriately to as close to its optimum summertime declination as possible, but in most cases, these collectors are placed wherever there is roof space available that has exposure to the sun. Unlike the flat plate or evacuated tube collectors, the polymers of pool collectors have a low thermal conductivity. They operate on a low temperature differential (\u0394T), and a high volume of water must pass through them in order to increase the water\u2019s temperature by a degree or two. Consequently, much more surface or \u201cwetted\u201d area is needed than the other two types of collectors mentioned in earlier text. Those operate on a much higher \u0394T and so require less surface area and volume. Swimming pool collectors operate by passing the pool water directly through them, so there is no need for any type of heat exchanger.\r\n

Solar Collector Performance<\/h2>\r\nSolar system designers look to collector manufacturers for data that reflects the output of their products. The makers of solar collectors have data available that express the thermal efficiency of the collector as a numerical value. This value is a ratio of the instantaneous heat output of the collector divided by the rate solar radiation strikes the panel. This is similar to the expression of thermal efficiency used by boiler manufacturers, in that it states the desired output quantity (collected heat) as a percentage of the required input quantity (solar \u201cfuel\u201d). However, unlike a boiler, there are more conditions that can change, which will affect the collector\u2019s thermal efficiency, such as fluid inlet temperature, the ambient air temperature and the intensity of the solar radiation striking it. These conditions can be plotted on a graph, with the horizontal axis reflecting the \u201cinlet fluid parameter\u201d, expressed as the result of the following equation: [latex]p=\\dfrac{{T}_{i}-{T}{a}}{I}[\/latex] where:\r\n