Ηλιακή Ενέργεια
Technical part: Thermal solar
MAIN COMPONENTS AND WORKING
MAIN COMPONENTS AND WORKING
Flowmeter: it allows to fit the flow to the requirements of the installation, by a 3-way ball valve. If the valve is in the closed position the flow is cut off, and it is possible to use the side tap to fill the plant. There is also another side tap, to drain the plant.
The proximity of the two taps helps these operations minimizing the distance between the filling and the draining. The flow rate is measured and shown by the special sliding cursor: the measurement is immediate thanks to the proximity to the regulation valve.
| (1) - Filling the installation: Remove the plugs from the side valves and connect the hose unions. Close the ball valve and open the side filling and draining valves. | (2) - Starting the installation working: Open the ball valve and close the side filling and draining valves. Remove the hose unions and screw the plugs. To avoid any casual opening of the side valves, it is better to stop the levers in the close position, as shown here aside. | (3) - Regulate the flow rate using the regulation rod until the right flow rate is shown. N.B.The flow rate is shown taking as reference the lower edge of the sliding cursor (see picture). |
| “Solar” checkball: It is included into the ball valve. It ensures the seal and low head losses. To exclude the checkball valve, for instance in case of emptying, rotate the handle by 45° clockwise. Security unit: The security unit, CE and TÜV approved, protects the installation from the overpressures. It is calibrated at 6 bar, over this pressure the security unit starts. It is also provided with a manometer and with a connection to the expansion vessel by a 3/4” flexible kit. |
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Model with the air vent: The air vent is a device that devides continually the air that can be in circulation together with the fluid. The air goes to the upper part of the air vent and it can be eliminated through the special drain while the installation is working. Unscrew the knurled metal ring lock for not more than half turn. This operation has to be done at intervals.
A careful planning allowed to reduce the headlosses of the air vent, getting a Kvs value 14.
 The incessant search for the quality pushed BRV to make a test of the air vent a report of which is available on request. |
THE PLANNING OF A SOLAR THERMAL INSTALLATION
The size of a solar installation is fundamentally different from the size of a traditional heating installation. The sun does not supply the whole necessary energy: only a part of it.
A proper energy storage will make up for the lacking of irradiation during the short periods, while during the long periods it will be necesary to turn to an auxiliary heat source.
It is important to know which part of the thermic requirement the solar installation is able to satisfy. The part of the usable energy collected depends on several parameters, first of all on the efficiency of the solar collectors.
This efficiency is related to the features of the collector (optical properties, insulation), to the temperature of utilization, to the inclination and the orientation of the collector, to the incoming solar radiation, to the outside temperature, to the speed of the wind. The efficiency of the solar collector is determined as the ratio between the usable energy collected Fr and the solar radiation cutting on the plane Iβ .
The usable energy can be calculated as the difference between the absorbed and the lost energy, taking into consideration the product transmissibility-absorption τα and the coefficient of thermic leakage Uc.
In conclusion the instantaneous efficiency of a solar collector can be couched in that way:
A proper energy storage will make up for the lacking of irradiation during the short periods, while during the long periods it will be necesary to turn to an auxiliary heat source.
It is important to know which part of the thermic requirement the solar installation is able to satisfy. The part of the usable energy collected depends on several parameters, first of all on the efficiency of the solar collectors.
This efficiency is related to the features of the collector (optical properties, insulation), to the temperature of utilization, to the inclination and the orientation of the collector, to the incoming solar radiation, to the outside temperature, to the speed of the wind. The efficiency of the solar collector is determined as the ratio between the usable energy collected Fr and the solar radiation cutting on the plane Iβ .
The usable energy can be calculated as the difference between the absorbed and the lost energy, taking into consideration the product transmissibility-absorption τα and the coefficient of thermic leakage Uc.
In conclusion the instantaneous efficiency of a solar collector can be couched in that way:
where Ti is the inlet fluid temperature and Ta is the ambient temperature.
All the collectors are tested under working conditions and the testing points are tranferred on the diagram:
All the collectors are tested under working conditions and the testing points are tranferred on the diagram:
By connecting the points we obtain the instantaneous efficiency straight line (Draw. 1)
Draw. 1 - Efficiency straight line of the distributor
The incoming solar radiation on the collector directed towards the equator and inclined of a β angle can be calculated as
800 W/m2 (* see notes). From the diagram it is clear that Ta (f.i. 10°C) and Ti being low (f.i. 26°C) the efficiency is:
otherwise, being the Ti high (f.i. 80°C) η ≅ 0,4.
Obviously there are two working points with very different efficiencies. For this reason it is necessary to take into consideration an intermediate working point (by convention 0,04 → η ≅ 0,62 as shown in Draw.1). According to these remarks the energy, that is really tapped from the fluid and brought to the installation, is only the 62% (Draw. 2).
Therefore the solar collector must provide a thermic capacity qa of above 500 W every m2 of tapping surface. It is advisable that, at the outlet of the collector the temperature Tu is not 6-9 K more than the inlet temperature.
If we consider that the specific heat of the fluid is equal to c=4000 J/kg K the flow rate of the collector is:
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Draw. 2 - Picture of a solar installation
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(*) Note:The density of the average power of the solar radiation outside the earth’s atmosphere is about 1367 W/m2. On the earth’s surface the maximum power is hardly ever more than 1100 W/m2, owing to the filter effect of the atmospheric components (gas, vapour, atmospheric dust) that absorb and disperse a part of the energy. More realistically, in the sizings, it is usual to assume from an average limit radiation of800 W/m2 up to a maximum limit radiation of1000 W/m2, taking into consideration several pejorative factors that can reduce the radiation absorbed by the solar collector. |
Therefore the solar collector must provide a thermic capacity qa of above 500 W every m2 of tapping surface. It is advisable that, at the outlet of the collector the temperature Tu is not 6-9 K more than the inlet temperature.
If we consider that the specific heat of the fluid is equal to c=4000 J/kg K the flow rate of the collector is:
Our solar pump units are provided with six different models of flowmeter: for small size installations 1÷6 l/min and 2÷12 l/min (360 l/h and 720 l/h); for medium size installations 8÷28 l/min and 8÷38 l/min (1700 l/h and 2300 l/h); for high flow installations 5÷42 l/min and 20÷70 l/min (2500 and 4200 l/h). To make an example, in the first case it is possible to install up to 8,5 m2of solar collectors, in the second case up to 17 m2, ecc.
Planning a solar installation it is very important to calculate the headlosses caused by the friction resistance of the fluid. It is necessary to know the headlosses of all the components of the installation. More than the solar pumping station we must take into consideration the heat exchanger inside the storage tank, the solar collectors and the pipe fittings. The headlosses are connected to the flow rate.
If for example we consider an installation of 22,5 m2. Therefore qt is: qt = 16 l/min ≅ 1000 kg/h.
Considering this datum the headlosses will be the following.
As concerns the headlosses of the heat exchanger, the manufacturer should give this value.
In the absence of definite data, taking into consideration a coil of proper size (section and length) we can consider the following ∆ps = 200 mm H2O.
The same for the solar collectors: even for them we consider a headloss of about 75 mm/m2.
Therefore: ∆pc = 75 × 22,5 = 1600 mm H2O.
The headlosses due to the pipe fittings, if for instance there is a copper pipe 22×1 on two lengths of 20 m each, are easily calculable by using the diagram of the Draw. 3, taking into consideration an increase of 25%, due to localized headlosses (bends and all kinds of pipe fittings).
Planning a solar installation it is very important to calculate the headlosses caused by the friction resistance of the fluid. It is necessary to know the headlosses of all the components of the installation. More than the solar pumping station we must take into consideration the heat exchanger inside the storage tank, the solar collectors and the pipe fittings. The headlosses are connected to the flow rate.
If for example we consider an installation of 22,5 m2. Therefore qt is: qt = 16 l/min ≅ 1000 kg/h.
Considering this datum the headlosses will be the following.
As concerns the headlosses of the heat exchanger, the manufacturer should give this value.
In the absence of definite data, taking into consideration a coil of proper size (section and length) we can consider the following ∆ps = 200 mm H2O.
The same for the solar collectors: even for them we consider a headloss of about 75 mm/m2.
Therefore: ∆pc = 75 × 22,5 = 1600 mm H2O.
The headlosses due to the pipe fittings, if for instance there is a copper pipe 22×1 on two lengths of 20 m each, are easily calculable by using the diagram of the Draw. 3, taking into consideration an increase of 25%, due to localized headlosses (bends and all kinds of pipe fittings).
Draw. 3 - Headlosses of the copper pipes
∆pt = (40 × 30) + 25% = 1500 mm H2O
The total headloss up to here calculated brings to the follwing value
∆p = ∆ps + ∆pc + ∆pt = 200 + 1600 + 1500 = 3300 mm H2O
At this point it is necessary to consider the presence of the solar station, to define the appropriate model of circulating pump which has to be used. Taking into consideration a qt always 1000 l/h and using, for example, a S2 Solar 3 8-28 l/min
(480-1680 l/h) solar station, its total headloss is ≅ 400 mmH2O ≅ 0,4 mH2O (draw. 4). Altogether the headloss is ≅ 3700 mmH2O ≅ 3,7 mH2O.
Draw. 4 - Typical diagrams of the pumping stations and of the circulating pumps
Click on the drawing to download pdf version in high-res; click here to download DXF/DWG version of the drawing
The model of circulating pump to be used is determined by the typical curve that is getting the closest to the calculated working point by excess; therefore the choice is for a circulating pump with a maximum nominal lifting of 6 m. So there is a margin to adjust its perfomance to the features of the installation. Operating the speed switch of the circulating pump (f.i. from III to II) or the flow regulator, it is possible to take the working back to the flow value previously determined: 1000 kg/h≅ 16 l/min.
The reliability of a solar thermal installation depends on the quality and on the life of the components and of the used materials. Of course you must be sure that all the materials conform with the plan and with the prescriptions of the manufacturer. Of course you must be sure that all the materials conform with the plan and with the prescriptions of the manufacturer. It is also better to verify the accuracy of the course of the pipes as concerns the balance of the installation; on this purpose a test of the compensation of the circuit must be done.
Then it is necessary to pay attention to the regulation of the plant, by checking that the collector sensor is correctly connected, the storage tank sensor is sufficiently dipped, the controller has been installed following the instructions.
The working tests usually foresee a circulation test of the fluid and a wet seal test.
The late regulations concerning the energy saving and the obligatoriness of the use of the alternative energy establish the check of the installation even in the case of a solar plant.
The thermic check of a solar installation is made to see the efficiency and the quantity of energy transferable to the users. The data to be taken into consideration for this check are the following:
The reliability of a solar thermal installation depends on the quality and on the life of the components and of the used materials. Of course you must be sure that all the materials conform with the plan and with the prescriptions of the manufacturer. Of course you must be sure that all the materials conform with the plan and with the prescriptions of the manufacturer. It is also better to verify the accuracy of the course of the pipes as concerns the balance of the installation; on this purpose a test of the compensation of the circuit must be done.
Then it is necessary to pay attention to the regulation of the plant, by checking that the collector sensor is correctly connected, the storage tank sensor is sufficiently dipped, the controller has been installed following the instructions.
The working tests usually foresee a circulation test of the fluid and a wet seal test.
The late regulations concerning the energy saving and the obligatoriness of the use of the alternative energy establish the check of the installation even in the case of a solar plant.
The thermic check of a solar installation is made to see the efficiency and the quantity of energy transferable to the users. The data to be taken into consideration for this check are the following:
- The inlet and the outlet fluid temperature of the solar collectors;
- The inlet and the outlet fluid temperature of the heat exchanger, filling side (domestic and heating);
- The fluid flow in the solar circuit and in the filling circuit.
The average efficiency of the solar installation ηm can be calculated as follows:
where Qu = qm × c × ∆t is the power espressed in [kW]; His the solar energy incident on the solar collector during the determined time [kJ/m2 · periodo]; Ac is the area of the tapping surface.
| Some remarks on the “High Flow” and “Low Flow” systems
According to the working conditions the solar installations can be fundamentally classified in two kinds: high flow e low flow; the element that decides the belongings to one or another category is the specific flow that is circulating into the solar collectors. In the first case it is about 0,5÷0,85 l/(min×m2), while in the second case it is about 0,25÷0,35 l/(min×m2). To do a general sizing like the one of the previous example, it is necessary to take into consideration that, starting from the available tapping surface (therefore from the real power supplied by the collectors) the choice of one or another technology brings to get a big ∆Tdifference in the exchanger: the high flow installations are working with a maximum 10 K meanwhile in the low flow installations the ∆T is up to 25 K. Starting from the above considerations and taking as exemplifying values of specific flow respectively 0,7 l/(min×m2) and 0,3 l/(min×m2) for the two system technologies, the table at side shows the maximum producible powers according to the different “sizes” of the installation. |
| The sizing described in the previous pages is pertinent to a high flow installation. If, on the contrary, it had opted for a low flow system, it would have been necessary to reconsider also all the section of the calculation concerning the estimation of the headlosses and the consequent selection of the circulating pump. The high flow systems are mainly used, meanwhile low flow, hanks to the high ΔT peculiar of this system, it is possible to get good results in case they want to push significantly the stratification of the water tank. |
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