Solar Energy and Thermal Energy Storage –
presentation of situation in Slovenia and Turkey
Halime Paksoy1, Uros Strith2, Hunay Evliya1, Vincenc Butala2,
Bekir Turgut1, Rok Stropnik2
1. Çukurova University Adana, Turkey
2. University of Ljubljana, Slovenia
Abstract:The paper presents the energy situation in two mediterranean countries and the way
towards sustainable development where thermal storage is one of the most important
technologies for efficient utilisation of solar energy.
Turkey has an increasing energy demand that is dominantly met by imported fossil fuel
resources. This brings several bottlenecks to the economy, energy security, diversity and
stability and environmental factors. Being a country in the EU integration process, Turkey has
to adapt her energy policies to meet the targets set by the EU. Policies and measures brought
recently by EU set binding targets for the year 2020 to reduce greenhouse gas emissions by 20
%, to ensure 20% of renewable energy sources in the EU energy mix and to reduce EU global
primary energy use by 20%. The adaptation activities to meet these very challenging targets of
the EU will help alleviate the current energy problems of the country of about 70 million.
Turkey has yet to exploit her abundant solar resources better as part of these activities.
Energy use in Slovenia is increasing and is based on fossil fuels, nuclear and renewable energy
sources. As an EU country Slovenia had to take all the EU Directives into consideration where
renewables must have a significant share. Among renewable energy sources the greatest shares
have hydro energy and biomass. Although the potential of solar energy is quite high, the
utilisation of this energy is still very low. Use of thermal energy storage technologies is the way
for increasing the share of solar energy in Slovenia. This is also one of the goals of “Green
paper” for Slovenian National Energy Program which is now in the phase of acceptance.
Key Words: Thermal Energy Storage, Solar Energy, Slovenia, Turkey,
1. INTRODUCTION
Sun is a continuous energy source of the earth. Solar energy is so abundant that one hour of
sun shine on earth covers world energy demand for an entire year [1]. One of the weaknesses
of this eternal energy supply is its intermittent characteristic. Therefore, solar energy needs to
be stored for efficient utilization. Thermal energy storage (TES) technologies, which are used
to close the gap between supply and demand of such intermittent resources, will be a solution
to solve this problem [2, 3]. Thermal energy storage is realized as a result of the change in
internal energy of any material. TES concepts involve utilization of one or a combination of
the following heats:



Sensible heat
Latent heat
Chemical reaction heat
Duration of the storage can be short or diurnal (day/night) long or seasonal (summer/winter).
For seasonal storage (summer/winter), underground thermal energy storage (UTES) is the
only option currently used in solar plants, e.g. in aquifers. For short term applications such
us sanitary hot water preparation thermal energy storage in phase change materials (PCM)
and thermo-chemical reactions are preferred [4]. This paper attempts to give an overview of
solar energy potential and the recent storage technologies used in solar applications in Turkey
and Slovenia.
2. TURKEY
2.1 Current energy situation in Turkey
Turkey is the one of the rapid developing countries in the world. Recently Turkey has
population of 70 million with 1.8% growth rate. Annual income is approximately 9.000
USD/capita in 2010, which is 3 times as much in 2000. Additionally, due to population
increase, economic and industrial development, energy consumption also rises in Turkey
every year (see Figure 1). Industrial energy consumption showed nearly 6 times increase in 35
years and also residential energy consumption has increased 3 times in the last 35 years.
Turkey mostly uses imported fossil fuel resources to cover her energy demand (See Figures 1
and 2).
Figure 1: Breakdown of consumption of energy sources according to sector in Turkey [5].
Figure 1 shows that, electricity is one of the most important energy sources of industrial
and residential consumption in Turkey. Figure 2 indicates electricity generation by fuel
resources. As one can see from Figure 2, natural gas share in Turkey has significantly
increased in electricity production in recent years. In accordance with the increasing
natural gas consumption, hydro power share in total electricity production in Turkey is
reduced to about 20% in the last 10 years. This also brings several bottlenecks of
economics, energy security, diversity, stability and environmental factors [1]. As an
impressive example of economic burden; Turkey has paid about 24 billion USD for
imported fossil fuels in 2009. Consequently, the Ministry of Energy and Natural Sources
of Turkey started to emphasize importance of energy conservation in her policies by
setting goals in different sectors. Ministry’s official estimated target is to reach an energysaving potential of 30% in the residential consumption, 20% in the industry, and 15% in
the transportation. Solar energy storage applications could decrease use of fossil fuels.
Figure 2: Electricity generation by fuel sources of Turkey [5].
2.2. Solar energy potential in Turkey
Turkey is located in a relatively advantageous geographical location for solar energy. Figure 3
shows annual solar radiation of Turkey. Especially Mediterranean and Aegean Sea coast has a
very high potential for utilization of solar energy.
Figure 3: Solar radiation of Turkey [5].
Turkey's average annual total sunshine duration is calculated as 2.640 hours (daily total is 7.2
hours), and average total radiation as 1.311 kWh/m²-year (daily total is 3.6 kWh/m²). Solar
energy potential is calculated as 380 billion kWh/year (See Table 1) Table 1 gives monthly
solar energy rate and sunshine duration. Regarding solar radiation potential in 25 EU member
and 5 candidate countries, Turkey takes place in the top five. [7].
Table 1: Monthly Average Solar Potential of Turkey [8].
Months
January
February
March
April
May
June
July
August
September
October
November
December
Total
Average
Monthly Total Solar Energy
(kWh/m2-month)
51.75
63.27
96.65
122.23
153.86
168.75
175.38
158.40
123.28
89.90
60.82
46.87
1311
3.6 kWh/m2-day
Sunshine Duration
(hours/month)
103
115
165
197
273
325
365
343
280
214
157
103
2640
7.2 hours/day
2.3. Laws and legislations on renewable energy in Turkey
Turkey has recently enacted its first renewable energy law in 2005 (Law No. 5346). The aim
of this law’s is to encourage the use of renewable energy resources (wind, solar, geothermal,
biomass etc.) for the generation of electrical energy. After this law has been enacted, 37
winds, 16 hydroelectric, 4 geothermal and 3 biomass licenses have been given by the Turkish
government [9]. It is a good start but not sufficient. Recently, 8% of total electricity
production comes from renewable energy sources in EU. EU commission’s target is to
increase this share to 20% till 2020. In order to achieve this target, Turkish current renewable
energy law is not sufficient, so a new and more effective renewable energy law has been
prepared in 2008. The Turkish government is now in the process of developing this new
renewable energy law, which is designed to encourage further investment and development in
the sector. This new legislation is expected to come into force in the next few months.
2.4. R&D on solar heating and cooling in Turkey
Solar heating and cooling applications are still under research and development stage in
Turkey. There are quite a few applications in recent years. Solar energy is stored either in
Aquifers and/or boreholes to provide greenhouse and residential heating [10]. Also few
commercial projects on hotel cooling with solar energy along the Mediterranean Coast of
Turkey are in operation. R &D projects are currently going on in 3 different cities in Turkey
on residential solar air-conditioning .Those projects are located in Ankara [8], Kayseri [11]
and Diyarbakır [12]. Diyarbakır solar house has started to operate in 2008 and currently %50
energy saving has been calculated [12]. These projects belong to Government, University and
Municipality respectively.
2.5. Solar energy for domestic hot water production in Turkey
Main solar energy utilization in Turkey is the flat plate collectors in the domestic hot water
systems. Turkey is one of the leading countries in the world with a total installed capacity of
8.2 million m² collector area (0.1 m2/capita) as of the systems are mostly used in Aegean and
Mediterranean regions (. Total energy production equals to 290 000 TOE (ton oil
equivalent). [8].
The solar collector industry is well developed with high quality manufacturing and export
capacity. The number of companies is around 100. Annual manufacturing capacity is 750 000
m²[8]. The contribution of solar collectors to the primary energy production is given in Table
2 [8].
Table 2: Contribution of solar collectors to the primary energy production in Turkey
Year
1998
1999
2000
2001
Solar Energy Contribution
(thousand TOE )
210
236
262
290
3. SLOVENIA
3.1. General about energy in Slovenia
Energy use in Slovenia is increasing in the past decades and is based on solid fuels, nuclear
energy and renewable sources of energy. Slovenia, as a member of European Union, had to
adopt all European guidelines and directives on energy especially for promotion of renewable
energy sources.
Energy use in Slovenia is followed statistically by the office of energetic in the ministry for
economy [13]. Use of primary energy was 273.7 PJ in 1997 . It consisted of oil products,
nuclear energies, natural gas, coals, hydro energy and renewable sources. Largest share of
103.1 PJ belonged to oil products, trace coal 62.3 PJ and then nuclear energy 51.6 PJ.
Consumption of primary energy was 305.7 PJ in 2007. In the last ten years energy
consumption increased for 32 PJ, which represents 11.7 % growth. Oil products, coal and
hydro energy have not perceptibly increased. Consumption increased in nuclear energy (11.5
PJ), natural gas (11.1 PJ) and renewable sources of energy (7.3 PJ). Data can be seen on
figure 4. Consumption of final energy in Slovenia was 207.7 PJ in 2007. Oil products
presented the largest share (101.3 PJ), electricity (47.7 PJ), natural gas (30 PJ), renewable
sources (17 PJ) and others.
Final energy in Slovenia for
the year 2007 (207,7PJ)
Primary energy in Slovenia
for the year 2007 (305,7PJ)
350
350
300
300
62,1
nuclear
42,5
earth gas
250
200
earth gas
200
103,9
coal
oil
47,7
PJ
PJ
oil
150
electricity
250
150
coal
30
water
100
50
66,5
11,7
0
district heating
100
renewables
renewables
101,3
50
19
0
4
17
7,1
Figure 4: Energy in Slovenia (year 2007): a) primary energy, b) final energy
Final energy in Slovenia is divided into three major groups: traffic, industry and households.
In traffic oil products (75 PJ), in industry electricity (26.9 PJ) and natural gas (25.5 PJ), in
households electricity (20.1 PJ), oil products (18.2 PJ) and renewable sources of energy (13.6
PJ) prevail. Largest share of renewable sources is being used for households, oil products for
traffic, electricity has large share within industry as well as in households.
Renewables;
13,6
Hauseholds
District h.;
2,5
Electricity ;
20,1
Industry
Electricity ;
26,9
Renewables;
3,4
District h.;
Earth gas;
4,6
4,5
Oil; 18,2
Earth gas;
25,5
Oil; 7,5
Coal; 4
Figure 5: Final energy in Slovenia (PJ): left in households, right in industry
3.2. Renewable sources of energy
Renewable sources of energy (RES) are important source of primary energy in Slovenia,
increasing of their share is one of the priorities of energy and environmental policies of the
state. Keeping in mind, that Slovenia imports around 70% of entire primary energy,
renewable sources of energy are national and strategic sources of energy, as well as their
social and environmental effects.
Slovenia has likewise good or as a whole better natural potential for the use of renewable
sources of energy in comparison to others EU states, because it is 54 % covered with forests,
which ranks Slovenia in European summit. Use of renewable energy sources with numerous
advantages over conventional sources of energy is that it contributes to reduce the dependence
on energy import, increases safety of supplies, the use of energy efficiently, allows creating
new working places and contributes to the enhancement of local country development of the
region.
Share of RES (including large hydroelectric power stations) in energy balance of Slovenia in
the year 2000 represents 9.2 % of all primary energy. Largest share represents energy of
biomass (wood and wood waste) and hydroelectric power station, followed by geothermal and
solar energy. With 9.2 % share of renewable sources (biomass 3.9 %) in primary energy
balance, Slovenia was on fifth place in EU (average in European Union was 5.5 %).
Share of RES in energy balance of Slovenia in the year 2007 mounted to 10.1 % of all
primary energy and it is increasing. Largest share belongs to energy of biomass (wood and
wood waste) (17.95 PJ) and to hydroelectric power (11.8 PJ), biogas(1 PJ) and solar energy
(0.1PJ) follows these two sources.
The sahre of renew able energy sources in Slovenia 2007 [PJ]
Biomass; 17,95
PV; 0,02
;Small hydro
11,8
biogas; 1
Wind; 0
Solar; 0,1
Figure 6: Share [PJ] of renewable sources of energy in Slovenia (2007)
3.3. Solar energy and its storage
In Slovenia 100.000 m2 of solar energy collectors or 0.05 m2 per inhabitant are installed, if
some data has been considered as a base ( Switzerland 0.02 m2, Austria 0.09 m2). Only shortterm thermal energy storage is now being used in households in Slovenia in larger scales .
These are used for preparing of sanitary hot water and for heating. On the seasonal storage
(aquifers) in Slovenia we still do not have a operating system, where we would be using heat
from aquifer in a renewable manner.
Measurements of environmental data have been realized in Slovenia for many years. Among
others measurements, solar radiation is one of the most predominant. Data are given in the
book Sončno obsevanje v Sloveniji (Solar radiation in Slovenia) 14. From the results we can
conclude that solar radiation is non-uniform in year cycle. The use of solar energy is therefore
closely connected to energy storage. Average day values are presented in table 1 for the
towns: Ajdovščina, Brnik, Novo Mesto, Koper, Maribor and Ljubljana.
Table 1: Data for average day solar radiation in Slovenia
Place
average day solar radiation
Ajdovščina
3210 Wh/m2day
Brnik
2950 Wh/m2day
Novo Mesto
3030 Wh/m2day
Koper
3400 Wh/m2day
Maribor
3010 Wh/m2day
Ljubljana
2960 Wh/m2dan
Figure 7 presents annual solar radiation in Slovenia:
Figure 7:Potential of annual solar radiation in Slovenia [15]
Theoretical potential of solar radiation in Slovenia can be calculated related to the area of the
State to 93700 PJ/a [16].
The potential must be reduced because only one part of Slovenia can be covered with
technological devices for solar energy conversion into other energies. We must exclude
forests, fields, rivers and roads. Therefore the total potential of solar radiation on horizontal
area which can be used for heat and electricity conversion is 19200 PJ/a [16].
Technical potential of solar radiation with consideration of all roofs in houses is 300 PJ/a
[16].
3.4. Researches, grants and action plan
Slovenia took part in the past in some international projects in the field of solar energy and
storage are as follows:
Solarge, Solair and RES-e regions.
Solarge was a European co-operation project to open up markets for large solar thermal
plants, focusing on installations from 30 m2 collector surface area upwards for e.g. hot water
supply, heating system support and cooling for multi-family buildings, hotels, public and
social buildings (for example old people’s homes and hospitals) [17].
Solair was to promote and to strengthen the use of solar air-conditioning systems. They are
neither well-established nor well-known in Europe, even though they offer considerable
potential and smart energetic solutions [18].
The project Res-e regions aimed at boosting electricity production from renewable energy
sources and the use of green electricity in 11 European regions by defining concrete regional
RES-e targets and developing and implementing regional RES-e strategies [19].
This year EKO FUND released a call of grants for non-repayable financial stimulations
for new investment of use of renewable sources of energy and larger energy efficiencies of
residential buildings [20]. Non-repayable financial stimulations are given to systems for use
of renewable sources of energy and larger energy efficiency of residential buildings on range
of Republic of Slovenia for new investments. For solar energy stimulations covers:
- purchase and installation of solar energy collectors;
- purchase and installation of thermal storage;
- intermediate connections, pumps, control and protective elements of system.
Height of non-repayable financial stimulation is up to 25 % of confessed costs of investment
and solar energies are awarded for at most 20 m2 of collectors, however not more than:
- 150 € on m2 of net surface at systems with flat collectors and
- 200 € on m2 of net surface at systems with vacuum collectors.
Figure 8 presents number of subsidized systems during the years:
Num ber of subsidised system s
Number of systems
1400
1200
1000
800
600
400
200
0
2002
2003
2004
2005
2006
2007
Year
Figure 8: Number of subsidized systems in Slovenia
Ministry of Economy prepared the first draft of the National Action Plan for renewable
energy sources for the period 2010-2020 (AN RES) on the basis of the Directive
2009/28/EC and the EC Commission Decision No: C (2009) 5174. For Slovenia it is expected
to achieve at least 25% share of renewable energy sources in gross final energy consumption
by 2020. Solar energy and storage in AN RES have an important place. In order for Slovenia
to achieve these goals, the following actions will be used:
a) programs of support
b) measures of cooperation with member states and third countries.
3. CONCLUSIONS AND RECOMMENDATIONS
For efficient utilization of solar energy, compact and cost-effective storage systems with high
energy storage density are essential. There is an urgent need to exploit the solar energy to
meet the growing energy demand and to sustain the life on earth. Solar energy is abundant in
Turkey and in Slovenia, but current applications are limited to solar domestic hot water
utilization. R&D on other solar applications is increasing in recent years.
Once solar systems are mass produced like conventional, fossil energy systems, and
integrated into buildings, thus replacing conventional systems they will become competitive.
A corrective pricing mechanism, such as a carbon tax, will also help competitiveness.
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