Solar thermal systems: what they are and how they work? Guide to the technical characteristics, operating principles, components and costs
Solar thermal energy is a technology used for a long time (since the early 800's) for the production of hot water. Solar thermal energy is the ideal resource to reduce the consumption of conventional energy through the use of an inexhaustible source: the sun.
The device base: "solar collector" is constituted by a black body absorbing within which a fluid can flow (with the function of capturing the energy radiated by the sun through the dark surface and transfer it in the form of thermal energy to the fluid) and a selective transparent cover on the part exposed to the sun (with the function of limiting losses by radiation towards the external environment), all enclosed in a container suitably isolated on the side walls and on the wall opposite to the reception of the radiation.
The solar collectors are connected together in series and in parallel so as to be able to produce significant quantities of hot water at a temperature between 50 ° C and 160 ° C. One square meter of solar panel can heat to a temperature of 45/60 ° C up to 300 l / day, depending on the weather. A solar thermal system consists of the following units:
1 or more collectors of selective titanium with prismatic tempered glass;
1 water storage tank.
In the case of forced circulation systems, plus the following:
1 hydraulic circuit;
1 electronic control system
These systems are reliable technology and competitive in the market and are widely used for the following purposes:
water heating for domestic, hotel and hospital;
heating of shower water (bathing, camping, etc.).
space heating;
heating water for low temperature processes;
drying of food and agricultural products;
cooling systems for buildings (still too expensive).
Solar thermal systems - Core Technologies
The solar heat at low temperature consists of three basic technologies:
Plastic panels
The first solution is characterized by a lower cost and is suitable for use summer, since the absence of glass roofing involves losses by convection too high for use with low outdoor temperatures. The water to be heated passes through the panel directly, avoiding the costs and complications in the system of the exchanger. It is therefore the ideal solution for bathing establishments, camps, outdoor swimming pools and the residences of summer vacation.
Flat plate collectors
Flat-plate collectors are the most common technology and more adaptable. Compared to the plastic ones offer a good performance throughout the year. From a constructive point of view are available various solutions that are characterized by the selectivity of the absorber plate, for the materials (copper, stainless steel and anodized aluminum) and to be suitable for use in forced circulation systems or natural (less expensive, more reliable, but less integrated with the architectural structures from an aesthetic point of view, because the storage tank must be positioned higher than the panel and in the immediate vicinity). The dimensions, while being present on the market particular solutions, usually provide for a footprint near the classic 100x200 cm2.
Vacuum manifolds
The evacuated tube collectors have the best performance in all seasons (about a 15-20% increase in energy production), thanks to the substantial cancellation of the losses by convection. The cost greater than the solution level, however, it recommends the adoption only in special cases (higher water temperatures and / or cold climate). They are in most cases of tubular shape, allowing the optimum inclination of the absorber plate, even if arranged in horizontal or vertical surfaces.
From the architectural point of view there are several examples of good achievements even in the case of pitched roofs. This usually involves the use of the forced circulation and therefore to a greater complexity of the plant. It should be said that now the technology is proven and reliable, if followed through periodic maintenance recommended by the manufacturer.
Solar thermal systems - Components Technical
In general, the facilities used consist of the following components:
solar panels
reservoir for the accumulation of hot water
other auxiliary components (ECUs control, circulation pumps, hydraulic and electrical connections, etc ...)
Their operation is expected that the circulation system to transfer the heat produced by the solar panels to the point of use or storage. Usually the heat carrier (typically a liquid) is water which is sometimes added an antifreeze solution to prevent freezing in cold weather.
Technically solar thermal systems are distinguished:
natural circulation systems
forced circulation systems
In the first circulation of the carrier fluid is physically activated to the effect "heating", in seconds the circulation is forced with the use of specific pumps.
Natural circulation systems
In such systems the heated water in the solar panel expands and salt in the storage tank, and was replaced by cold water that goes down into the tank. Such systems tend to be cheaper (compared to "siblings" forced circulation) being devoid of pumps. The natural circulation systems are used primarily in areas with higher solar incidence. The insulated tank accumulates the necessary heat. The heat is transported by the carrier fluid to the reservoir through a specific circuit, the fluid is never in contact with drinking water. The fluid in the panels, while heating with solar radiation, becomes lighter and rises in the tank where it transfers its heat to the domestic water through the metal walls of a heat exchanger, losing heat the fluid cools and returns downward.
Below is shown a simple diagram that describes the connection of the various components for a natural circulation system:
Forced circulation system
The forced circulation systems are a bit 'more' complex than natural circulation 'cause the fluid in the primary circuit is driven by a pump to the solar panels. E 'need to install a forced circulation system where the water storage tank can not be positioned at a higher level compared to the solar panels. The tank can be installed, usually, in a local acting as central heat.
Below is shown a diagram that describes the connection of the various components for a forced air circulation system:
The system works like this: the sun with the temperature of the fluid coming out of the collector exceeds that of the kettle and the controller switches the pump which circulates the heat transfer fluid by transferring heat from the collectors to the water in the kettle, and after a sunny day the boiler, having accumulated the energy captured, it's hot. If solar heat is not enough to re-ignite the pump switches off in more favorable conditions, at sunset the fluid output of the collector cools and the pump stops. The hot water stored in the boiler remains available to users in temperature for a few days. Generally, these systems have a double boiler heat exchanger: the solar calendar located below and to integrate placed higher. The stratification of the hot water in the water heater allows you to make the most of solar energy since, if it is necessary to integrate the boiler operates on a limited amount of water.
Why install a plant for domestic use?
Through solar energy can make up 85 to 90% of the per capita water needs for domestic use of the water, or to wash and wash clothes and dishes (the last generation appliances provide the ability to use pre-heated water). Calculate the annual savings resulting from the use of a solar energy system for heating water for domestic use is not straightforward since there is in the game several factors including some (such as individual habits) are highly variable; wanting to face a speech "rough" is possible, however, to estimate a savings of about 170-220 m3 per capita per year, including in this estimate also the savings resulting from the complete shutdown of the boiler, including pilot flame for the period between April and October. These savings for a family of 4 with a methane gas, resulting in actual savings amounted to € 210-270 per year, net of substantial effects such as inflation and the usual increase in the price of gas the same . In this respect it is not possible not to add, although difficult to quantify in economic terms, the general environmental benefit resulting from the non-issuance of about 1000-1500 kg of CO2.
How much does a solar system of this kind?
The price of a plant for the production of hot water for sanitary use domestic for a family of 4 people varies in function of both the desired amount of water is the complexity of installation of the same. The plant can be amortized within 9-10 years. Considering, then, the possibility to claim back income tax as required by applicable law, the payback time would be reduced proportionately decreasing up to 5-6 years.
Environmental Benefits
In addition to the potential economic benefits of which we have already discussed, we must not omit the substantial environmental benefits that the spread of these plants would produce. To quantify these benefits is possible to assess the amount of carbon dioxide (CO2) emitted into the atmosphere by a variety of ways commonly used to produce hot water (electric water heater, gas boiler, solar panels). Incidentally it is worth remembering that the carbon dioxide and 'considered one of the causes of excessive overheating of the planet earth. To produce the necessary hot water to its requirements, a family of 4 with an electric water heater uses about 7.7 kWh of electrical energy per day. To produce 1 kWh with a thermoelectric power plant will emit about 0.7 kg of CO2, a water heater, then, is responsible for about 5.4 kg of CO2 per day. A natural gas boiler, instead, uses about 0.9 cubic meters of fuel per day per family. As in the combustion of methane produces about 1.96 kg of CO2 per cubic meter, the daily emission is equal to 1.77kg of CO2. With the solar panels do not have any emission of CO2 or other air pollutants such as dust, nitrogen oxides and sulfur oxides. In any case, the panels can also be used to supplement the boiler or water heater with gas emission reductions of 60%.
20/05/2012
----------------------------------------
Translated via software
----------------------------------------
Source:
Italian version of ReteIngegneri.it