The study was accomplished in 2010. The applied research methods are based on the analysis of the studies by other authors, industry experts, statistic data and the (unpublished) information provided by the Latvian district heating merchant “xxx”. Spearman’s rank correlation coefficient was used for evaluating data regularity. In 2020 the energy produced from renewable sources (Directive 2009/28/EC) in total energy consumption in Latvia has to reach 40%. Such an objective can be achieved by promoting the use of biomass at cogeneration (CHP) plants, the potential of which is the district heat supply system. Wood as an energy resource in its structure accounts for only 25-30%; therefore, the primary condition is the introduction of modern technology which would provide the possibility to diversify energy resources. The calculations confirm that the realised amount of thermal energy indirectly affects profit; directly it is affected by the rate of thermal energy, so the structure of use of energy in heat supply enterprises is associated with lowering the cost of (restrictive) factors - the energy cost, specific production technologies, energy efficiency. Currently, the primary condition of heating companies is to find such energy resource(s) to ensure the approved schedule of temperature and thermal load. The next determining factor is the energy price. In assessing the factors that will affect energy usage in the future, the main strategic principle of energy consumption will be linked to limiting the growth or even reducing heat tariff, by realising optimal efficiency and minimal environmental pollution during energy transmission.

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.

One can reasonably argue that issues related to the increased use of renewable energy resources in the energy production processes in Latvia, are at the forefront and will remain there in the future. This relates to the aspect that Latvia is not rich in non-renewable energy resources (around 70% of total primary energy consumption in Latvia is ensured by import, which can lead to undesired effects in many areas), but at the same time, there are available renewable energy resources in Latvia, with an untapped potential to be recognized. In particular this applies to fuel wood, which is already (year 2012) the most important domestic fuel in Latvia. In this context it is important to emphasize that, according to the particular study results, if unexpected socio-economic developments do not take place, raw wood material resources required for different types of fuel wood production in Latvia should be available in the same amount as it is now if not more. As for increasing the amount of fuel wood use in Latvia, an enormous ‘potential’ can be seen in general use boiler houses, where there are currently no technological limitations to utilize this ‘potential’. General use cogeneration plants can be recognized as an even greater ‘potential’ for greater use of fuel wood in Latvia, but given the circumstances of energy supply in Latvia, the ‘potential’ is currently available on a very limited basis. At the same time it is important to note that both of these ‘potentials’ could be significantly reduced in the next few years.

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.

The paper discusses heat transfer and the step response of a low-power induction motor to no-load mode under fan cooled and naturally cooled conditions. In Latvia University of Agriculture (LUA) in the electric drive laboratory the experimental tests were performed on a 1.1 kW totally enclosed fan cooled three phase induction motor with a fan mounted on a shaft for fan cooled conditions and with a fan taken off for naturally cooled conditions. The transient temperatures are measured in nine points of the stator end windings and in two points of the stator frame using thermocouples. Temperature is measured by using K type thermocouples and Pico-Log TC – 08 data logger. The current and voltage are measured by using Simple Logger II L562 two channel data logger. Measurement data are processed and archived using data loggers Pico-Log Recorder, Simple Logger II and Data View software. The experimental test results show that ventilation plays an essential role in the heating process of small power induction motors. Mathematical and virtual models of induction motor windings heating are represented to simulate the heating process of induction motor under no-load mode and different cooling conditions.

Wood construction elements are widely used in buildings and they can be used in construction elements such as roofs, floorings, windows and doors. As wood is combustible material, there are lot of fire regulations that should be fulfilled at the design stage of any building project. In Latvia, as in some other countries usage of wood in building constructions is limited. Wood materials are an ecologically friendly material and a renewable resource, and its usage should be promoted. The paper presents an overview of heat distribution in wood floorings exposed to standard heat fluxes which is different compared to wood members exposed to high heat radiation. Fire spread and self-ignition investigation on wood floorings is carried out in this research. The results show different kinds of the temperature distribution in the cross section of the wood member cross section depending on heat radiation and location of the temperature measurement point. Temperature distributions in wood during fire can be completely different in dependence on heat radiation and cross section of the wood member. The wood materials in floorings do not reach the self-ignition temperature during standard heat radiation.

In conditions where induction motors are frequently started, overloaded and used in high inertia applications with long starting times, supplied from frequency converter, a temperature protection system are more reliable to protect induction motor stator winding against thermal overloads. There are different types of temperature protections - thermostat, PTC thermistor, resistance temperature detector (RTD) and thermocouples, so it is important to know the properties of each type to choose an adequate protection system. Analyses of temperature sensor properties and their advantages and disadvantages show, that PTC thermistor is a cost-effective temperature protection solution, but for medium and high voltage induction motor protection RTD are commonly used. A virtual model has been represented to simulate the temperature sensor thermal time constant under different thermal conductivity and thickness of winding magnet wire insulation.

It is very important to use the exact investment model for energy enterprises. Giant investment is necessary for establishing continued energy systems using renewable energy resources. Demand of the heat energy usually is higher in a winter period compare to the summer time. Program combinations of different heating modus are researched using EnergyPRO.