In the previous article we already mentioned productivity of a solar collector, however there are still some factors which influence operability of solar collectors in гелиосистеме.
As it was already mentioned, key parameters for an assessment of efficiency of solar collectors are passport data, as a rule, specified in certificates, namely: optical efficiency (??) and factors of thermal losses (and? and and?).
? – Efficiency of a solar collector;
? T – a difference of temperature of air and an absorber of a solar collector;
Е–intensity of sunlight;
On this formula approximate calculations of productivity гелиосистемы are usually made. Calculations not always display a full picture, however with their help it is possible to judge quite precisely average development of heat by solar collectors for the chosen period of time.
Efficiency of solar collectors in practice
In practice business can be a little differently. One of factors influencing productivity is the volume expense of the heat-carrier in solar collectors. Value of optical efficiency and factor of thermal losses of a solar collector given in certificates corresponds to a certain expense of the heat-carrier in a collector. For example, according to certified laboratory SPF the solar collector of the Viessmann Vitosol 200-F trademark (number of the certificate of C513) has a heat-carrier expense at test of equal 200 l/h. At recalculation on 1 m? the expense is equal to the aperture area of 40,6 l/h of m?. Thus there is an essential difference between this value and recommended value of the producer of 25 l/h of m?. In certain cases, the difference between these indicators can will cause a stir in 3-4 times.
Certainly, such distinctions in speed of a stream, play important role in efficiency of a solar collector at work in actual practice. Decrease in speed of a stream of the heat-carrier influences heat-carrier temperature on an exit of their collectors. The speed of the heat-carrier is lower, the heat-carrier temperature is higher. More high temperature on an exit from a collector is often wrongly perceived, as an absolute indicator of efficiency гелиосистемы. At present among producers the tendency to reduction of speed of a channel of the heat-carrier is traced. Now optimum value in the majority of recommendations of producers makes 25-30 l/h of m? unlike recommendations of 5 summer prescription of 40-60 l/h of m?. It gives the chance to reduce hydraulic connection and allows to use less powerful pumps and smaller diameters of pipes in гелиосистеме.
However, at decrease in value of a volume expense of the heat-carrier the temperature of an absorber of a solar collector increases also that in turn increases thermal losses of a solar collector in environment. Thus, the part of useful heat is simply lost, without reaching an accumulator tank. Therefore, more high temperature on an exit from collectors is not an indicator of high efficiency of a solar collector.
For an example we will consider efficiency of solar collectors depending on speed of a stream of the heat-carrier. For research solar collectors were chosen: And – HEWALEX KS 2000 TP and In – VIESSMANN Vitosol 200-F. Calculations are given proceeding their values of solar intensity of 800 W/m?.
Efficiency of solar collectors depending on speed of a stream of the heat-carrier and an absorber design
At an expense bigger expense of the heat-carrier (60 l/h of m?) productivity of a solar collector is higher for 5 %.
In a case with vacuum collectors a picture approximately same. And in certain cases development of thermal energy at a low volume expense of the heat-carrier, even is less than for flat collectors.
Difference in productivity of vacuum tubular collectors
Probably, the reason of understating of speed of a stream of liquid is the desire to show as though more effective work of collectors, entering thereby into errors of the users who are wrongly considering more high temperature as an indicator of working capacity.
The second factor to which often, do not attach significance, is a way of connection of collector groups. Let’s consider on an example of connection of group of three solar collectors with a various design of an absorber and at various speed of a stream of the heat-carrier.
Dependence of development of thermal energy on connection of solar collectors in group
In the first option absorbers are connected as though consistently and consequently the heat-carrier temperature in each following collector is higher. In the second case the temperature is distributed evenly. At connection still the difference becomes even more obvious than bigger number of collectors.
Solar collectors with a type absorber a meander
Collector with a design of the meander type it is not recommended to establish more than 5 pieces in one row.
Thus, efficiency of a solar collector in practice can considerably will cause a stir from settlement sizes. It is necessary to consider such parameters as an expense of the heat-carrier and connection of collector groups, and as some other recommendations.