Efficiency is the measure of an activity performance and depicts how well the input (material, energy, effort in general) is transformed in useful output. In other words, the efficiency represents the share of input which is transformed in a desired result. For instance, in case of a natural gas fired steam boiler, 80% efficiency indicates that 80% of the fuel energy consumed by the boiler is transformed in useful heat contained into the steam produced, while the remaining 20% are losses (loss of heat through the boiler’s casing, with flue gases and others).

Useful energy is the amount of energy delivered to the final consumer (technological process, building, etc.), while the Final energy comprises the amount of energy necessary in order to cover the consumption of useful energy (as an example, the amount of final energy can be estimated by dividing the amount of useful energy to the overall efficiency of the conversion system).

Primary energy is a form of energy found in the environment, in natural form, not subjected to any human manipulation, conversion or transformation. It is contained in fuels in natural state or in other forms. Further, it is transformed within different conversion processes in a form usable by the society (refined fuel, electrical energy, heat, etc.). Recalculation (expression) of an energy form usable by the society in units of primary energy is performed by applying (multiplying to) the respective primary energy factor.

The primary energy factor is the multiple applied in order to express the forms of energy usable by the society (fuel conditioned for usage, electricity, etc.) in primary energy. E. g., the primary energy factor of 1.2 for 1 unit of natural gas which is finally consumed indicates that for this 1 unit there were utilized 1.2 units of gas in natural state (before refining, losses of transportation & distribution, etc.).

A primary energy factor of 3.8 for electricity indicates that for 1 unit of electricity finally delivered at the consumer there were consumed 3.8 units of primary energy (e.g. natural gas from nature extracted, refined, transported, burned at the power station and transformed in steam heat, further in mechanical energy in turbines which drive electric generators, then finally the electricity transported and distributed to the consumer, each stage registering respective losses).

Therefore, the primary energy factor brings to the lowest level, to a so called common denominator, different forms of energy usable by the society, for a clear comparison.

As a result of different processes, there can occur emissions of gases with greenhouse effect (GHG). Even in cases when there is consumed a form of energy (or material) which does not suppose burning, this consumption may be a cause of GHG emissions (e.g.: 1-consumption of electricity implies GHG emissions at the power station it was generated or 2-usage of some materials which may result in methane leakage, etc.). As different GHG have different magnitude of the greenhouse effect, for comparability, their effect is expressed (brought to a common denominator) into the effect of carbon dioxide (which has not the highest greenhouse effect but a large share in the atmosphere).

CO₂ emissions factor has the same role as the primary energy factor, but it indicates what quantity of CO₂-equivalent emissions occurred for the production and supply of a unit of energy/fuel used at the consumer (e.g. 0.6 kgCO₂ for 1 kWh of electricity supplied to the consumer).

Such ventilation system implies partial recovery of heat from the air evacuated from a room/building and its subsequent transfer to the incoming (outdoor) air (either via heat exchange surfaces, on one side flowing the leaving air and on the other – the incoming air, or via regenerative systems, or applying heat pumps, etc.). To this end, a share of the leaving air heat is not wasted, thus, the amount of heat required in order to heat up the incoming air decreases. The same applies for the cooling season, when the leaving indoor air partially cools the incoming (outdoor) hotter air.

LED lamps are lights based on light-emitting diodes. These are characterized by a high luminance efficacy (lumen/watt), long operation period and reduced maintenance costs, but require higher initial investment. From an energetic and economic standpoint, such lamps are a significantly better alternative for many types of lamps currently used.

A variable speed drive (VSD) is a device which regulates the frequency and torque of the electric motor, by adjusting the frequency and voltage of the input current. Thereby, the operation (load) of the motor is adjusted to the needs (load) of the process, thus reducing (often significantly!) the consumption of electricity.

Co-generation and tri-generation comprise simultaneous production of two, respectively three forms of energy within the same installation/plant. For instance, co-generation of electricity and heat with an installation based on internal combustion engine (operated on fuel, the engine produces mechanical energy which drives the electrical generator, while the heat of the cooling oil and exhaust gases is used to produce hot water). Or tri-generation of electricity, heat and cooling; in this case, to the installation mentioned above there can be added an absorption machine which produces cooling using a part of the heat provided by the engine.

An energy management measure is an action, either organizational, or instructional, change of behavior/culture of use and consumption, etc. with the aim to rationalize and, thus, reduce energy consumption in an enterprise, building, etc. An energy management system is a more broad approach and comprises a complex (set) of energy management measures and, also, can include measures for monitoring, verification and control.

A renewable source of energy is a natural resource which regenerates, as a consequence of permanent natural processes, in such a way that it is capable to replenish faster (or to keep up with) the rate of utilization, in a finite time comparable to the human timescale. Examples of such resources are the solar radiation, wind, natural water flows, biomass, geothermal heat, etc.

The advantage of energy use from such resources is the fact that these are practically inexhaustible (on the human scale), while their impact on the environment is, usually, minimal.

A heat pump is an installation which extracts („pumps”) heat from a low potential (temperature) source (e.g. soil), increases the potential (temperature) of this heat and transfers it to a heat sink (consumer, e.g. a building, process, etc.). As naturally heat flows from an object with a higher temperature to another one with a lower temperature, and not vice versa, for the operation of a heat pump there is required additional external energy (electrical, mechanical, others).

The main performance indicator of a heat pump is the coefficient of performance (COP), which depicts how many units of heat it „pumps” from the source with the consumption of 1 unit of additional energy, e.g. a COP of 4 of an electricity driven geothermal heat pump indicates that it extracts 4 units of heat from the source (soil), consuming 1 unit of energy (electricity). Despite de fact that, usually, heat pumps require a higher investment, at certain COP levels (depending on the real situation and the premise/process considered) these become a more efficient alternative to traditionally used sources (boilers, heaters, etc.), leading to reduced energy bills.

A solar thermal collector is an installation designed for capturing solar radiation and transforming it into heat, in order to produce hot water. There can be different types of solar thermal collectors, such as plate, evacuated tubes, etc., with different advantages and disadvantages and range of application.

A photovoltaic (PV) system is a system based of PV panels which transforms captured solar energy into electricity, by means of photoelectric effect. There are PV systems with different types of PV modules (mono- and poly-crystalline, amorphous, etc.), different configurations and auxiliary components, each with associated advantages and disadvantages as well as different suitability to the specific situation and location.

Despite that PV systems require rather high investment, often (case by case detailed assessment required) the levelized cost of produced electricity over the lifetime of the system is significantly lower than of the electricity from the public grid.

A wind turbine is an installation intended for transforming the kinetic energy of air movement (wind) into electricity, as a result of the rotation of turbine’s blades by the wind and subsequent drive of the electric generator.

Hence, the wind turbine is in fact a windmill, but certainly more sophisticated, with safe and control systems and a much higher efficiency of conversion of the kinetic energy of wind into mechanical energy (and, subsequently, into electricity).

Biogas is a bio-fuel, resulted from the decomposition of organic material, and considered as a renewable resource. It comprises a mixture of gases released as a result of decaying process, in the absence of air, of (but not limited to) agricultural wastes, manure, sewage, household waste.

It means obtaining of biomass fuel in special geometrical forms and specific characteristics (density, humidity, ash content), by means of chopping and pressing biomass material (such as branches, wood chips, sawdust, walnut bark, sunflowers husks, etc.) and subsequent usage as fuel. Production process can be of a different complexity, with several additional steps, such as sorting, drying and others.

Pellets have a cylindrical form, with a diameter of 5-8 mm and a length of 40-50 mm. Briquettes have a cylindrical form, usually with edges (corners), with a diameter of 50-60 mm and a length of 200-250 mm. Dimensions are indicative.