Theoretical and Experimental Investigations of Cylindrical Air-Heating Solar Collector

The main use of solar energy in Latvia is for water heating in solar collectors. But solar energy can be used for heating of water or air not only with solar collectors, but also with photovoltaic (PV) panels. Here electrical heaters have been used; from the electro-technical viewpoint they are of active resistance with power factor cosϕ = 1. Such kind of water heating has some advantages in comparison with solar collectors. In case of solar collectors heating goes on only when the temperature of the heat carrier exceeds that of the heated water, therefore, the efficiency of heating strongly depends on the temperature of the heated water. Power of the electric heater does not depend on the temperature of the water. In case of solar collectors heated water (or heat carrier) must be transported at rather a large distance from the collector to the place where the heated water is used, leading to large heat loss. In case of PV panels the electric energy has been transported with much less loss. Because of non-linear load curves of PV panels and dependence of the shape of the curve on solar irradiance and the air temperature, the energy received from the panel depends on the resistance of the heater. Traditionally voltage converters with the function of searching of point of maximal power have been used. Such converters are very expensive. Therefore, we propose to use constant resistance with the value appropriate to the used PV panel and to the mean weather conditions and solar irradiance. Measurements of heating of water have been done. It is found that the lower the solar irradiance, the larger resistance gives better results. In our situation the load resistance 4.6 Ω gives the best results, while from the producer's information at high solar irradiances 3.8 Ω load resistance is the best.

Section: Methodsmentioning

confidence: 99%

Improvement of efficiency of PV panels for water heating by changing heater resistance

Pelece

Ziemelis

Putans

et al. 2019

Section: Introductionmentioning

confidence: 92%

“…In northern countries, for example in Latvia, solar energy has been used [4], but mainly in the same kinds as in the southern ones without taking into account the particularities of use of solar energy in higher latitudes. Among other peculiarities [5], in this case the most important role is played by lower ambient temperature.Although efficiency of photovoltaic is only about 0.2 (or even 0.15 for large-scale production), but of solar collectors it is up to 0.75 (or approximately 0.5 for large-scale production), there only the same device has been taken into account. For solar collectors heat losses take place also in one or two (depending on construction) heat exchangers, where more effective heat transfer is possible if temperature of heat carrying fluid is high, and in the transferring process from the place of producing to the place of consuming (especially, if the distance is long and/or the temperature of ambient air is low), where losses are directly proportional to the temperature of the heated water.…”

mentioning

confidence: 92%

Water heating with tracking to sun solar batteries

Pelece¹,

Putans²,

Ziemelis³

et al. 2017

Abstract.Traditionally it has been considered that solar collectors are more effective for water heating than solar batteries (PV) because of changing of energy from heat to electricity and then back to heat in the case of batteries. But there are some aspects why solar batteries can be more effective for water heating than solar collectors, especially in northern countries, where ambient temperature is low. The first of them is negative impact of temperature difference between the absorber of the solar collector and ambient air on the efficiency of the solar collector. Therefore, efficiency of the solar collector importantly decreases with increasing of the temperature of the obtained hot water. The efficiency of solar batteries only slightly depends on temperature and, opposite to the case of solar collectors, is higher at lower ambient temperature. The same situation is at the heat exchanger traditionally used with the solar collector: the amount of heat given from the heat carrier to usable water depends on the temperature difference between them. Electricity from solar batteries heats water independently on its temperature. Also tracking to sun is more complicated for solar collectors than for solar batteries. Water heating using tracking to sun solar batteries has been studied in this work. Two photovoltaic panels Solet P6.60-WF-250 with surface area 3.25 m 2 each, mounted on a dual axis tracking device, were used for water heating with electrical resistance heaters. Measurements were carried out from 1 of August till 22 of October 2016. The results show that the measured power of batteries on sunny October day is approximately 500 W and 28 l reservoir without heat insulation of water has been heated up to 50...70 °C, or by 30 °C during 2h what corresponds to 490 W received power. An opposite dependence of the photovoltaic efficiency on ambient temperature also has been observed.Keywords: solar energy, solar batteries, tracking to sun, water heating. IntroductionSolar collectors are used mainly for water heating. Only one study [1] has been found where photovoltaic are used together with the conventional heating system. It can be explained by particularities in use of solar radiation in different latitudes. The main solar energy using countries are in low latitudes: the leading countries are China, Turkey, Brazil and India [2]. Therefore, also almost all studies in this branch are concentrated in these countries, only one article [3] from several hundred has been found related to use of solar energy in latitudes higher than 50º. In northern countries, for example in Latvia, solar energy has been used [4], but mainly in the same kinds as in the southern ones without taking into account the particularities of use of solar energy in higher latitudes. Among other peculiarities [5], in this case the most important role is played by lower ambient temperature.Although efficiency of photovoltaic is only about 0.2 (or even 0.15 for large-scale production), but of solar collectors it is up to 0.75 (or approximately ...

“…As explained in [5], efficiency can be illustrated by plot of daily energy gain from the solar panel via daily sum of solar energy received by the panel. Figure 2 shows the temperature of water heated with the vacuum tube collector Vitosol 200 SD2.…”

Section: Fig 1 Pv Panels (A) With Solar Tracker (B)mentioning

confidence: 99%

Use of PV panels for water heating in Ulbroka research centre

Pelece

Ziemelis

Putans

et al. 2018

Solar energy has been widely used not only in southern countries, but also in Latvia. In Latvia solar energy has been used mainly for heating of water and/or air in summer. In the Ulbroka Research Centre starting with summer of 2008 the vacuum tube solar collector Vitosol 200 SD2 is used for water heating in summer, when the central heating system does not work. Area of this collector is 3m 2 , and it works with 300 l hot water reservoir Vitocell B-100 CVB. Such collector satisfied the demand for hot water till 2013, when severe hail damaged the vacuum tubes of the collector. 9 tubes from 30 were broken. Now we propose photovoltaic (PV) panels for water heating. PV panels have some advantages in comparison with solar collectors: there is no need of the complicated system of heat exchanging and transporting of the heat from the roof of the house, where the solar collector is situated, to the rooms, where the hot water is used; there cooling is not going on, when the sun is covered by clouds; the heating power does not depend on the temperature in the hot water reservoir; it is more simple to ensure tracking to the sun for PV panels than for the solar collector. Measurements of water heating capacity of both stationary and sun tracking PV panels have been done to find out the necessary area of PV panels to satisfy the demand for hot water in the Ulbroka Research Centre. It was obtained that two PV panels Solet 60.6-WF-250, mounted on the tracker ensuring all day perpendicular striking of direct solar beams on the panel's surface, produce in average 4.66 kWh daily in summer. Such amount of energy is sufficient to warm 150 l of water to 26 Celsius degrees.