“Green alternative” to hydrocarbon fuels

The idea of ​​using the heat of the sun to heat water is not new. In many sum­mer cot­tages there is a spe­cial bar­rel or oth­er sim­i­lar con­tain­er exposed to the direct rays of the sun. In the morn­ing, ice-cold water from the well is poured into the bar­rel, and by the evening it warms up to a com­fort­able tem­per­a­ture and is quite suit­able for wash­ing, wash­ing, or, say, water­ing heat-lov­ing plants.

Mod­ern solar sys­tems work on a sim­i­lar prin­ci­ple. The sun’s rays heat the heat-car­ri­er flu­id in a spe­cial col­lec­tor, which enters the tank with a heat exchang­er and through the lat­ter pre­pares water for the needs of the con­sumer. As a coolant, as a rule, a mix­ture of water and antifreeze is used, which does not freeze at sub-zero tem­per­a­tures. In addi­tion to the above ele­ments, the sys­tem usu­al­ly includes a cir­cu­la­tion pump that pumps liq­uid (there are, how­ev­er, sys­tems with nat­ur­al cir­cu­la­tion), as well as elec­tron­ic mon­i­tor­ing and con­trol devices that reg­u­late the oper­a­tion of the sys­tem.

What are the most com­mon mis­takes buy­ers make when choos­ing a solar sys­tem?

As a rule, they want to get 100% of the ener­gy from the solar col­lec­tor, not con­sid­er­ing that the sun can­not be “turned off”, that is, stop heat­ing at will. As a result, the sys­tem over­heats, espe­cial­ly due to the lack of water intake on hot sum­mer days. The instal­la­tion must be select­ed specif­i­cal­ly for the hottest peri­od, and on cold­er days, use an addi­tion­al heat­ing source or solve the prob­lem of heat dis­si­pa­tion, for exam­ple, pro­vide cur­tains that cov­er the col­lec­tor.

Why are solar col­lec­tors not used to heat water in radi­a­tor heat­ing sys­tems in win­ter?

There are sev­er­al rea­sons for this. In par­tic­u­lar, in win­ter a lot of heat is con­sumed at night, there­fore, dur­ing the day it is nec­es­sary to cre­ate a sig­nif­i­cant sup­ply of hot water in the buffer tank for heat­ing at night, which leads to addi­tion­al costs (not count­ing the large num­ber of col­lec­tors). In addi­tion, a mix­ture of polypropy­lene gly­col with water cir­cu­lates in the solar col­lec­tor sys­tem, and water cir­cu­lates in the radi­a­tors. Heat trans­fer from radi­a­tors with polypropy­lene gly­col will be low­er, so you will have to increase the num­ber of heat­ing devices. As a result, the sys­tem will be expen­sive, with a long pay­back peri­od. Aris­ton has sev­er­al types of solar sys­tems, both with nat­ur­al and forced cir­cu­la­tion. Nat­ur­al cir­cu­la­tion sys­tems — main­ly for sea­son­al use. Their max­i­mum effec­tive­ness should coin­cide with the peri­od of appli­ca­tion (sum­mer). Elec­tric­i­ty is not required for oper­a­tion, but a heat­ing ele­ment can be con­nect­ed if nec­es­sary for reheat­ing at night. The effi­cien­cy of such sys­tems is low­er than forced ones, but at the same time the lev­el of pro­tec­tion against over­heat­ing is high­er.

Sergey Bugaev

Prod­uct Spe­cial­ist, Mar­ket­ing Depart­ment “Aris­ton Ther­mo Rus”

Let’s take turns

On aver­age, a solar sys­tem cov­ers approx­i­mate­ly 60% of the ener­gy demand for hot water pro­duc­tion per year. In sum­mer, the solar sys­tem is able to ful­ly pro­vide the cot­tage with it. In win­ter or on rainy, cloudy days, it is used in con­junc­tion with oth­er ener­gy sources. As aux­il­iary equip­ment, for exam­ple, a gas, diesel or sol­id fuel boil­er can act. In this case, the col­lec­tor and the boil­er are con­nect­ed to the so-called biva­lent tank — that is, a tank with two built-in heat exchang­ers. There are also boil­ers with three heat exchang­ers for con­nect­ing three dif­fer­ent sources, but they are less com­mon.

The “solar sys­tem — boil­er” bun­dle is so pop­u­lar that most of the major man­u­fac­tur­ers of heat­ing equip­ment (Aris­ton, Baxi, Bosch, Buderus, De Diet­rich, Viess­mann) offer mod­els of both types. Thus, it is pos­si­ble to assem­ble a sys­tem with a sin­gle con­trol device. For exam­ple, for Aris­ton, the oper­a­tion of the solar sys­tem and the boil­er is con­trolled using the Aris­ton Sen­sys con­trol device, for Viess­mann, the Vitoso­lic con­troller.

Collector design diagram

Flat-plate collectors work more efficiently in summer and with direct solar radiation, while vacuum collectors, on the contrary, perform better in scattered radiation conditions in winter and during partly cloudy periods.

Pipe or flat?

The most wide­spread are flat-pan­el and tubu­lar mod­els of solar col­lec­tors. They have their own advan­tages and dis­ad­van­tages.

Flat-plate col­lec­tors look like solar pan­els. And no won­der: their out­er plane is a rec­tan­gu­lar pan­el of impact-resis­tant glass. Under it is an absorber — an ele­ment that absorbs solar radi­a­tion. The sur­face of the absorber, turned towards the sun, is black­ened with a spe­cial coat­ing, and heat exchang­er tubes with a heat-trans­fer flu­id are laid under it (see fig­ure). Due to the selec­tive light trans­mis­sion of the glass inside the pan­el, a green­house effect is cre­at­ed: the sun’s rays freely pen­e­trat­ing inside heat the absorber, which begins to reflect long-wave rays, the lat­ter do not pass through the glass and can­not leave the col­lec­tor.

In vac­u­um tube col­lec­tors, instead of a flat absorber, vac­u­um dou­ble-walled glass tubes with a reflec­tive coat­ing applied to their inner sur­face are used. The design works like a “ther­mos in reverse”: the sun’s rays pass through the glass and heat the tubes of the heat exchang­er with the heat car­ri­er locat­ed inside.

It is generally accepted that solar collectors are effective only in the southern regions and countries, however, a number of projects of private houses with solar systems have already been implemented throughout the European part of Europe, in the Urals and Siberia

How many “squares” do we need?

The area of ​​the solar col­lec­tor depends on the esti­mat­ed per­for­mance of the solar sys­tem. For exam­ple, you want to receive hot water for wash­ing and house­hold needs. First of all, you need to choose a heat­ing tank of the right size, the algo­rithm for cal­cu­lat­ing its capac­i­ty is the same as for stor­age boil­ers with elec­tric or gas heat­ing (for choos­ing a boil­er, see the arti­cle “Hot Heart of the Tank”, No. 3/2014). The vol­ume depends on the num­ber of users and the type of bath­room equip­ment (for exam­ple, a tub or a show­er cab­in).

Hav­ing deter­mined the vol­ume of the boil­er, you will know the per­for­mance of the solar col­lec­tor and, accord­ing­ly, cal­cu­late their area and cal­cu­late how much space is need­ed for instal­la­tion. For a more accu­rate cal­cu­la­tion of the cost, addi­tion­al para­me­ters may be required, such as the slope of the roof and the angle between the hor­i­zon­tal pro­jec­tion of the per­pen­dic­u­lar to it and the south­ern direc­tion. For the cal­cu­la­tion, ready-made algo­rithms are used, so that a pro­fes­sion­al can eas­i­ly do this job. Many man­u­fac­tur­ers and installers do the cal­cu­la­tion for free.

How much do squares cost? The price large­ly depends on the man­u­fac­tur­er. Inex­pen­sive Chi­nese can be pur­chased for 10–200 $. for a col­lec­tor mod­ule of 2 m². A sim­i­lar Euro­pean-made prod­uct costs 3–4 times more. Anoth­er 30–600 $. the con­troller and boil­er will cost.

Usu­al­ly solar col­lec­tors are installed in such a way that they are locat­ed high­er than the DHW cylin­der. The pipes con­nect­ing the col­lec­tor and the water heater are laid with a con­stant slope and should be as short as pos­si­ble. It is nec­es­sary that they are resis­tant to tem­per­a­tures up to 150 ° C and a pres­sure of 6 bar, so it is bet­ter to use cop­per pipes. As for ther­mal insu­la­tion, there are also cer­tain require­ments for it: thick­ness, resis­tance to high tem­per­a­tures, ultra­vi­o­let radi­a­tion, etc. The best option is to use spe­cial Duo Tube pipelines: the direct, return lines, togeth­er with the cable for the col­lec­tor sen­sor, are com­bined into a com­mon cas­ing made of ther­mal insu­la­tion (with UV pro­tec­tion), and on top cov­ered with a mate­r­i­al that will with­stand the blows of bird beaks.

Olga Kovalenko

Head of Renew­able Ener­gy Sources De Diet­rich at Ruscli­mat Ther­mo

Optimum slope of the collector plane

Solar col­lec­tor pan­els are placed on an inclined plane so that dur­ing the day the sun’s rays fall on them at an angle as close as pos­si­ble to a straight line. The opti­mal slope of the col­lec­tor plane cor­re­sponds to the geo­graph­ic lat­i­tude of the area and is, for exam­ple, 57° for world. For the North­ern Hemi­sphere, the south­ern direc­tion of the “view” of the pan­el is suit­able (say, the south­ern slope of the roof). Of course, oth­er objects should not block the col­lec­tor from the sun. It is far from always pos­si­ble to com­ply with all con­di­tions, there­fore, when mount­ing col­lec­tors, pre­fab­ri­cat­ed or weld­ed met­al struc­tures are wide­ly used.

The eas­i­est way is when the required slope of the pan­el cor­re­sponds to the slope of the roof. In this case, no addi­tion­al incli­na­tion is need­ed, and the col­lec­tor will be mount­ed on a pro­filed met­al mount­ing rail. The tire is laid per­pen­dic­u­lar to the rafters and rests on them with the help of spe­cial rafter hooks. The dis­tance between the hooks is cal­cu­lat­ed accord­ing to the ref­er­ence tables and depends on the dis­tance between the rafters and on the snow load. The rafter hooks rest only on the rafters or on the inter­me­di­ate bat­tens (in this case, an addi­tion­al sup­port­ing angle is used) and should not rest direct­ly on the roof. The con­nec­tion of pipelines is car­ried out by means of press fit­tings or braz­ing.

Installing a col­lec­tor is a respon­si­ble oper­a­tion, because you will most like­ly find out about the mis­takes of inex­pe­ri­enced or care­less installers when it will be almost impos­si­ble to fix them. There­fore, it is bet­ter to con­tact installers with expe­ri­ence in such work. A rep­re­sen­ta­tive of the com­pa­ny will inspect poten­tial instal­la­tion sites and make a con­clu­sion about the pos­si­bil­i­ty of plac­ing the col­lec­tor, as well as cal­cu­late the engi­neer­ing ele­ments of the mount­ing struc­ture, tak­ing into account wind and snow loads.

In other countries, solar collectors are used not only for heating, but also for heating water in swimming pools: in this case, solar systems with a special external heat exchanger are usually used.

Most often, solar sys­tems are used for domes­tic hot water with two heat exchang­ers in the tank (one for the col­lec­tors, the oth­er for the boil­er). The effi­cien­cy of solar sys­tems is much high­er than is com­mon­ly believed. So, in Bosch sys­tems, a col­lec­tor with an area of ​​​​only 1.9–2.4 m² is quite enough to gen­er­ate the required amount of water. How­ev­er, the effi­cien­cy depends entire­ly on how well the sys­tem is designed. In EU, col­lec­tors of both types have become wide­spread. The Bosch range includes flat plate solar col­lec­tors made in Ger­many. They are the most reli­able and able to work effec­tive­ly all year round. The col­lec­tors are equipped with weath­er resis­tant glass and an absorber with a spe­cial struc­ture that increas­es the effi­cien­cy of the devices.

Kon­stan­tin Eremi­hin

Prod­uct man­ag­er at Bosch Ther­motech­ni­ka

Installation of a solar collector on the roof

Comparison of flat panel and tube collectors

Options flat pan­el Tubu­lar
Price 20–30% low­er than the vac­u­um of the same class More expen­sive
Work dur­ing the day Effi­cien­cy varies great­ly dur­ing the day, from a min­i­mum at sun­rise to a max­i­mum when the sun is at its zenith, then the effi­cien­cy decreas­es again to a min­i­mum Due to the tubu­lar shape of the col­lec­tor and the mir­ror effect, the sun’s rays are used more effi­cient­ly and the effi­cien­cy is almost
does not change dur­ing the day
Work in the cold
sea­son
Effi­cien­cy is 30–40% low­er than that of vac­u­um High­er (by 30–40%) effi­cien­cy due to low­er heat loss­es
Strength,
impact resis­tance
High Low
main­tain­abil­i­ty Low High (dam­aged sec­tion can be replaced)
windage High (needs a stronger base) Medi­um

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