Use of solar energy increases every year. In Latvia, solar energy is used mainly by solar collectors. The main part of the solar collector is the absorber, but not less important is the covering material which protects the absorber from the cooling impact of the wind. This cover must be transparent for solar radiation, but opaque for thermal radiation of the absorber, which is at greater wavelengths. Therefore it is important to measure absorption spectra of possible covering materials at visible and infrared wavelength ranges. Absorption spectra have been measured for several materials: glass, polythene, Plexiglas, and cells Plexiglas. Absorption spectra for all these materials are measured in three ranges: ultraviolet – visible (UV – VIS): 250 – 1000 nm; near infrared (NIR): 700 – 110 nm; infrared (IR): 1200 – 8000 nm. UV-VIS spectra with the ‘Ocean Optics’ device HR-4000 have been measured, but NIR and IR – with ‘Bruker’ Furje spectrometer EQUINOX 55. Evaluation of absorption spectra showed that the most suitable material (from the considered) for covering of solar collectors is Plexiglas.

Pollution of atmosphere and decrease of fossil fuels stimulates people to search for an alternative energy sources for production of energy, both electrical and heat. In Latvia it is possible to use almost all alternative energy sources for production of energy, including solar energy that becomes more relevant year by year. Solar radiation in direct way makes no hazardous, but, as the intensity of solar radiation in geographical location of Latvia is comparatively low, the utilization of it demands relatively expensive equipment. Production of energy from other sources of energy also demands certain investments as well as the purchase of fuel, maintenance of equipment and presence of different manipulators. In practice several types of solar collector constructions with efficiency from 30 up to 75% exist and expenses vary in the wide range. The average number of sunshine hours in Latvia is about 1800 hours yearly. Nevertheless solar collectors for water heating in Latvia are used. Mostly flat-plate collectors are used whose efficiency often is not sufficient and water has to be additionally warmed-up. The aim of our investigation is to study operation of evacuated tube collector in conditions characteristic to Latvia. It is stated that it is feasible to use evacuated tube collector for water heating in Latvia, especially from March till October. During winter period solar collector can be used as additional energy source for water heating.

The aim of investigations was to compare different absorber material efficiency to examine collector tracking the Sun and stationary collector efficiency as well as the efficiency of insulated and non-insulated collectors. The 0.1x0.5x1.0 meter long experimental solar collectors were constructed for investigations and different types of absorber materials were made. The manifold length, the Sun radiation effect on the degree of air heating was analyzed. ASHARE used standard 93-2003 is determined for insulated and non-insulated solar collectors and absorber effectiveness. We determined the influence of the Sun radiation on the air heating degree for those types of absorbers. The experimental data were measured and recorded in the electronic equipment REG (electronic metering and recording equipment of temperature, radiation and lighting). The collector covered material was a polystyrol plate and different absorbers. We compared insulated and non-insulated collectors to prove that the insulated collector is more effective. The insulated collector was made of the collector surfaces faced with cellular plastic two cm plates. Our task was to calculate the air heating solar collector efficiency. The collector tracking the Sun is approximately 30% more efficient than the same type of stationary collectors in operation time for 6 hours. Insulation increases solar panel efficiency especially in windy weather. The insulated collector demonstrates almost two times higher efficiency than the non-insulated collectors (up to 93%) with absorbent material steel tinplate on top.

In order to get maximum economic effect, when setting a solar collector in a peasant yard, it is important to choose the correct place. The yield of obtained heat energy will be at its maximum, if the temperature of surrounding air and solar radiation are higher by the relative humidity of the air - lower. To define these parameters, a special device 'Meteorological Device' (MD-4) was developed. The device was supplied with a mechanism which tracks the sun all day round. So the direct solar radiation on the surface perpendicular to the sun beams was measured. The second measuring of the solar radiation was made in relation to the surface, which was perpendicular to the sun beams only at the middle of a day time. For the measuring of the air temperature and relative humidity, corresponding sensors were used. In every 15 minutes the data of these four meteorological parameters were automatically measured and the results were saved into a logger - the device for data accumulation. After a certain period of time the information was brought into the memory of a computer and analysed.

This paper provides an overview of recent progress reached in semi-transparent photovoltaic systems (STPV), which are being assessed as a potential solution to enhance the productivity of plant grown in greenhouses. Utilizing this kind of renewable energy resources, relating with plant growing is attractive solution to increase sustainability for citizens. The aim of this study is to find out recent advances for application of various semi-transparent photovoltaic systems which can be integrated in greenhouses. Solar PVs are among dependable, mature and cost-effective renewable energy systems and solutions, which are promising for building integrated photovoltaics (BIPV) application. The main emerging photovoltaic candidates for BIPV are amorphous silicon, kesterite, chalcopyrite, CdTe, dye-sensitized, organic and perovskite based systems. A monographic study approach has been utilized in this investigation, in ordain to compile and analyse the photovoltaic systems for BIPV mainly investigating and comparing two main parameters: average visible transmittance (AVT) and power conversion efficiency (PCE). The rapid development of new materials and structures for the manufacture of semi-transparent solar panels allows a balance to be struck between AVT, PCE and a comparison of the reviewed materials indicates that organic and perovskite are the most promising for semi-transparent solar panel production and application in greenhouse constructions, based on their PCE and AVT results.

The energy-intensive timber industry enterprises have a significant role in Latvia’s manufacturing sector, contributing to the country’s sustainable economic development. They maintain the economic well-being of residents of Latvia’s rural regions and represent a substantial proportion of output value and export volume in the manufacturing sector and the state economy. Due to surging energy prices that negatively impact production costs, it is more difficult for enterprises to compete in foreign markets. The study aims to analyse energy consumption trends in timber industry enterprises in Latvia to propose activities for reducing energy costs, thus promoting the international competitiveness of timber industry enterprises. The analysis is based on data from Europe’s power market Nord Pool and databases of the Official Statistics Portal of Latvia. Methods used: statistical data analysis using time series analysis and expert interviews. The research results show that a significant threat to the competitiveness of enterprises is the increasing price of electricity, which is the second most consumed energy resource, accounting for nearly 15% of the total annual energy consumption of timber industry enterprises. To address high electricity costs, enterprises have the opportunity to install solar panels. Calculations show that, at an electricity price of 0.198 EUR per kWh, investments could lead to a five-year payback period with an ROI of 7.52%. At a lower electricity price of 0.13 EUR per kWh, the payback period would extend to ten years but still provide a reasonable ROI of 5.2%.