Written by : Andreas Gavrielatos , Mechanical & Aeronautical Engineer  

and Dimitris Tsiatsios, Chemical Engineer NTUA


The average building in Greece, covers the needs for heating and hot water boiler with an oil or gas and other energy needs (e.g. freezing) with electricity. It is now regarded as the building sector contributes significantly to environmental degradation.

The technology in recent years has made great strides in this area and there is now a multitude of options to meet the energy needs of a house in heating, cooling and hot water, with applications that are easy to use, environmentally friendly and cost effective.

Renewable energy sources which can be used in terms of residence are:


  • Biomass
  • Geothermal Energy
  • Solar Energy



The term biomass call any material derived directly or indirectly from the vegetable world. This includes: materials from natural ecosystems (forest wood), residues from agricultural and forestry processes (cake, cotton ginning waste, sawdust, straw, maize stems, stalks of cotton, branches of trees, animal waste), and biological part of urban sewage and garbage.

The energy of biomass is a secondary solar energy and is the result of photosynthetic activity of plants. Chlorophyll of plants transforms solar energy through a series of processes, using as main raw materials of carbon dioxide from the atmosphere and water and minerals from the soil. The burning of biomass has a zero balance of carbon dioxide (CO2) and does not contribute to global warming - because the amounts of CO2 released during combustion of biomass is the same as those absorbed by plants during their lifetime. Also, the minimal presence of sulfur in biomass contributes significantly to reducing emissions of sulfur dioxide (SO2), which is responsible for acid rain.

Globally, biomass could yield 9% of global primary energy and 24% of energy needs by 2020. 

Heating applications, besides the traditional firewood, wood chips are also used, as well as olive pits, sawdust and pellets


The firing takes place in modern boilers with high-tech auto fueling and electronically controlled air flow, which can produce more than 90% of the energy contained in biomass. Fireplaces and woodstoves are also used, with potential for producing hot water or air.



Geothermal energy is called the natural heat of the earth leakage from the hot interior of the planet to the surface. The center of the Earth temperature is 5.500°C, according to recent calculations, at its core. That is about as hot on the surface of the Sun. Even the upper three meters of the surface of the Earth remain nearly constant temperature of 10-16°C throughout the year. On average, the temperature beneath the surface rises by 3°C per 100 meters of depth.

The utilization of geothermal energy is important for humans to meet needs, as it is a practically inexhaustible source of energy. 

The term "geothermal air conditioning" means the use of constant temperature and heat capacity of the subsoil where the use of geothermal heat pump may transfer heat to and from the soil to produce energy for heating or cooling and hot water. 

In Greece, of course, depending on the region, there may be strong variations in temperature from summer heat (40-42°C), up to sub-zero temperatures in winter (-15°C). But few meters below the earth's surface the ground remains at a relatively constant temperature, independent of the temperature of the atmosphere throughout the year. In Greece, the annual mean temperature of the subsoil depth> 2 m varies in the range of 16-20°C, depending always on the latitude of each region.

This difference in temperature between the outside ambient air and the subsoil exploit the use of geothermal air conditioning system. Air conditioning systems which use the "Low depth geothermal energy", can draw heat from the subsoil to use for indoor heating (winter) or to remove heat from the building giving it to the underground (summer). Thus, the use of geothermal energy can contribute to energy saving up to 40% on heating and 50% - 70% for cooling. 


The methods of installing a geothermal system vary, depending on the terrain, the availability of the site, the existence or not of groundwater, the air conditioning requirements and other factors and therefore should be defined and designed for each case specifically. The geothermal systems are divided at open or closed systems : 



The closed loop ground heat exchanger consists of a network of buried polyethylene pipes. The circuit can be horizontal or vertical depending on how the pipes can be placed. The amount of available free space and the subsoil texture largely determine the type of geothermal heat exchanger.


Horizontal geothermal system

Horizontal type of installation is appropriate for residential applications where sufficient land is available. It requires digging in the soil surrounding the building in depth 0,80-1m. Horizontal ground heat exchanger tubes are usually made of high density linked polyethylene. These pipes run in the trench in circuits similar to the heating, which means collectors are driven to heat pump. The subsoil acts as a thermal energy storage which contributes significantly to the higher efficiency of the installation.

The required land is a function of thermal and cooling requirements of the building. For designing the geothermal heat exchanger, knowledge of soil temperature and thermal responses in-depth installation are needed. 




Vertical systems can be installed in facilities with limited space environments and the inability to absorb water from the aquifer. For a vertical system, boreholes are needed, located about 10m from each other. Into the boreholes a U-shaped pipe is placed inside. Then the holes are filled with Thermal mix (cement, bentonite). The number of wells needed depends from the total thermal power required. Their depth varies between 60-120m. 

Advantage of vertical systems is that their performance is almost constant throughout the year. 




Main feature is the abstraction and return of groundwater or surface waters and their heat exploitation. The design of the open circuit usually includes two boreholes: Production borehole, where the submersible pump is immersed and the disposal borehole. The water is pumped from the aquifer (groundwater, soil, sea, lake or river), passing through the heat pump which absorbs heat, it returns and is then reintroduced to the aquifer. This system is more economical in construction than the enclosed ground heat exchanger, but is only for areas with rich aquifer, and only when the lower level of abstraction from the borehole does not exceed 50 meters.

Because groundwater has a relatively constant temperature over the year, it represents an excellent source of heat.

The production borehole water should under existing law, after the heat recovery from the heat pump, be repaid in full in the aquifer from which it was drawn. The drilling of rejection must have the ability to accept the entire quantity of water passed through the heat pump. 



The transfer of energy from the building loads to soil or groundwater is accomplished through the geothermal heat pump, which produces cooling, heating and domestic hot water. Heat pumps can be linked to underfloor heating and cooling systems as well as Fan Coil heating and cooling systems. They can also be combined with an existing boiler or other renewable sources such as solar heating systems for support. 



Solar energy is characterized as all the different forms of energy taken from the sun. Such is light energy, thermal energy and radiant energy. Solar energy as a whole is practically inexhaustible, clean, soft and renewed.

Regarding the use of solar energy, this is divided into three categories of applications: passive solar systems, active solar systems and photovoltaic systems. 

Greece, is a country with plenty of sunshine and is ideal for using solar energy. The average annual sunshine in our country is about 3,000 hours per year. The installation of a 1m2 collector can save up to 500-650 KWh per year.

The solar thermal systems for combined operation to produce DHW and space heating can meet by 10% - 50% the needs for home heating and hot water use on an annual basis, depending on the installed solar collectors area, the buffer tanks’ volume, the meteorological data in the region and the characteristics of the house (size, quality of insulation, thermal requirements).




To combine the ideal implementation of the system, we must have well insulated houses with low temperatures heating systems (underfloor, Fan coils), while heating radiators expect a reduction in yield by 15%-20%. A big advantage of these systems is that they can be installed both in new and existing homes without special modifications.

Generally, these systems comprise the circuit of solar panels (power production), the buffer tank (stored energy), an auxiliary power system (electric boiler, oil boiler, biomass boiler, heat pump), a heating system (radiators, underfloor heating, Fan coils) and a control system (automation).

The operating principle of such a system is very simple and similar to a central heating system solar hot water 

The solar energy collected in solar panels is converted to heat and transported 

in a specially designed and constructed for this purpose buffer tank, which first heats up the space heating water and then the domestic hot water. If solar energy is not enough, then the auxiliary power system turns on and completes the required energy. With this method we can achieve significant savings in fuel (or current), while the whole heating thermodynamic circle is achieved in a manner which is environmentally friendly, particularly in a country like Greece, where the need for saving energy and reducing greenhouse gas emissions becomes increasingly urgent.

The proper operation of such a system and thus maximizing energy savings requires both the correct sizing of the system and also the selection of appropriate materials.


The solar panels should have high efficiency selective surface, since the system requires the maximum energy collection during the winter. 

One of the key elements of the solar heating system is the buffer tank, which is the "heart" of the system and must be specially designed and constructed for this purpose. The buffer tank should be well insulated and in particular to promote the stratification of the water temperature inside. 

The stratification of the reservoir leads to maximum system performance, minimum heat losses and maximum energy collected by solar panels.

Italian factory SICC manufactures COMBI tank which is specially designed and constructed for this purpose. It consists of two separate containers, one inside the other. The outside container (buffer tank) circulates the water for the heating system. Inside the tank inertia exists a second tank (domestic hot water tank), which is treated with high-tech material VITROFLEX, which makes it suitable for drinking water. 

Inside the buffer tank a heat exchanger is immersed, for the connection of the vessel with the solar panels. 



COMBI tank has strong insulation from hard polyurethane (70mm) to minimize heat loss.

The inner hot water container is equipped with electronic anodic protection of titanium anode. This system ensures the anti-corrosion protection due to electrolysis, it needs no maintenance and it is absolutely efficient with low power consumption.


YDROENERGIA , a technical company based in New Port Corfu, being a leader at systems involving renewable and alternative energy sources, undertakes the study, supply and installation of the appropriate heating system for each application.



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