Due to the ever increasing drive in technological and industrial developments coupled with increases in human population, environmental pollution is worsening. This phenomenon, alongside the rise in prices of fossil fuels has increased a large portion of current research towards new and improved renewable energy sources (Chiasson et al. 2000). For most populations across the world, whether low density (i.e. traditional rural) or high density (i.e. current or future cities), solar energy is considered as a primary energy source for the future however, there are issues with its supply and demand which serve to limit its usability. Storing solar energy in a suitable way can provide some level of equilibrium between supply and demand. Energy storage systems can improve utility of solar energy and also:
- contribute to energy conservation
- provide economical usage of fuel
- reduce the amount of wasted energy
Storage of solar energy in an appropriate way and developing systems to make it available on-demand must be a key mission of today’s energy researchers (Sharma et al. 2009). Fossil fuels, which many national energy networks, including Cyprus are largely dependent on, are now limited and are predicted to be an insufficient and expensive energy source in the future (Grätzel 2001). In addition to this, domestic energy demand continues to rise year on year. Increased energy demand is not only dependant on the rise in energy consumption, but also on environmental impacts such as climate change and atmospheric pollution (both are increasing).
Within this context, various international agreements such as the Kyoto Protocol seek to address this problem (Zhou et al. 2012). As a result of the increasing awareness on the importance of renewable sources for reducing dependency on fossil fuels, the EU commission aims to increase the share of renewables to 20 % by 2020 in member countries (Böhringer et al. 2009a; Böhringer et al. 2009b). Whilst Southern Cyprus is a member of EU, Northern Cyprus (NC) is not but there is still an onus on NC to develop infrastructure and strategies to increase usage of renewable sources as it has the potential to be a part of EU in the near future.
The island of Cyprus (see: Fig. 1) has a remarkable solar potential (see: Fig. 2) when compared with most of the developed EU countries such as Germany and UK. This abundance of dense solar radiation should be fully utilized, especially during the summer months when it is at its peak. Due to historically insufficient economical outlay, alongside the technical and managerial background of the country, usage of solar energy is largely limited to flat plate solar collector technology for water heating purposes (Ibrahim & Altunc 2012). Generally, these systems can only be used from March to November as the solar availability in winter is inadequate to heat the water to a usable level (>40 °C). There are some cases where small size water storage tanks are used to store heat sensibly however, these tanks are rarely insulated and thus provide little or no advantage in utilizing the solar resource effectively. In NC, it is traditionally direct electrical heaters that are employed for space heating purpose. Whilst water heating accounts for 45 % of end energy usage, space heating constitutes the second highest energy share (28 %) in the residential sector (Atikol & Güven 2003; Atikol 1996). The reason for this can be due the large total area of residential buildings which, in general, can be 150 to 200 m2 and usually consist of many uninsulated building components (i.e. walls, floors and roofs) with common usage of low efficiency single glazed windows.
There are currently no residential or commercial applications for utilizing solar energy for space heating during the winter months in NC (Evcil 2012) and, even with the abundance of energy during the summer period, it cannot be employed in winter for this purpose because of the seasonal mismatch. Thermal heat storage (THS) systems should be the main driving force for seasonal storage of solar energy and usage of it during winter conditions. This will significantly contribute to reducing the usage of electricity and thus fossil based fuels in the residential sector for space heating applications. THS systems bring the advantage of storing summer heat, densely with very low heat loss and space requirements (Aydin et al. 2015; Yu et al. 2013).
In this study, the suitability of THS for increasing the solar share in space heating applications for NC climate conditions is investigated. Within this context, firstly, energy trends in NC (past and future) are discussed. THS potential is evaluated through the analysis of material based experimental results (i.e. COP, output temperature, temperature lift and heat storage density) of a selected THS material called SIM-3a. The year round heating load of a selected building in NC with the solar heat gain of the same building from 8 m2 solar air collectors is simulated. In the final part of the present research, an integrated solar THS system applicable to NC conditions is proposed and discussed in terms of its technical and economic feasibility.
Energy trends in Northern Cyprus
North Cyprus is part of a small island located in the eastern Mediterranean Sea. According to the census carried out in 2006 by the NC government the population is circa 265,000 with a total number of residential customers of ≈ 80,000 (http://www.kibtek.com/Santrallar/santrallar.htm). The rate of increase in demand for electricity has doubled from ≈ 7 % in 2004 to > 30 % during recent years due to population and construction growth in all geographical regions. The total energy consumption, Ec in 2009/2010 for NC was 913 GWh whereas for 2011/2012 it was 1256 GWh corresponding to a 37 % increase as shown in Fig. 3.
By 2012 the installed capacity of power plants in NC was 329 MW with a total energy generation, Eg of 1349 GWh (http://www.kibtek.com/Santrallar/santrallar.htm). Electricity generation is spread between steam turbines (35 %) and diesel generators (65 %) where gas turbines are used as backup (http://www.kibtek.com/Santrallar/santrallar.htm). Recently, a large scale solar photovoltaic (PV) power plant was installed with the support of the EU however, utility of this power plant is still not sufficient due to the low generating efficiency (<10 %) of PV cells. This has been attributed to high ambient temperatures in summer and grid connection problems. The installed plant capacity, P and energy generation, Eg of all NC units for 2012 are presented in Fig. 4.
The residential sector in NC currently accounts for ≈ 35 % of total national energy consumption (see: Fig. 5a) with the share of water and space heating being the two heaviest loads at 45 % and 28 % respectively (see: Fig. 5b). In total, 73 % of the energy demand of a building in NC is thermal (i.e. space heating and hot water). As seen in Fig. 2, the remarkable solar potential of Cyprus could be utilized to reduce fossil fuel consumption for space heating and hot water purposes. Feasibility studies on thermal energy storage systems for NC should be carried out as this technology has the potential to store solar energy and increase the solar fraction for building thermal energy consumption. Furthermore thermochemical heat storage (THS) brings an advantage of storing solar energy in summer and using it in winter without any heat loss or degradation.
The high share of overall energy consumption attributed to the residential sector is primarily due to the dependency on fossil fuels for domestic activities such as space and water heating with the majority of water heating in NC provided by either electricity or Liquefied Petroleum Gas (LPG). Central or district heating systems are rarely used in NC as a high percent of buildings are detached and winter climate conditions are not extremely cold. For this reason mainly air sourced heat pumps (HP) or direct electrical heaters are used to provide heating to each room of a dwelling separately. Additionally, in some dwellings, LPG sourced single heating units are also used. Although wood burning stove or fireplaces were popular in the past, these units are currently rarely used due to health and safety issues, environmental pollution and user heavy operating conditions such as temperature control.
NC does not have any oil or natural gas based fossil sources and imports all fuels types required for electricity generation, residential and commercial needs from other countries. Increases in population and recent industrial developments have caused a significant rise in demand for electricity and fossil fuel consumption (http://www.kibtek.com/Santrallar/santrallar.htm). There has also been a sharp rise in electricity unit prices. In 1990 electricity was 0.0008 €/kWh whereas the current unit price of electricity is 0.22 €/kWh, representing an increase of 275 %. Energy consumption in residential buildings has also increased almost 4 fold from 112 GWh to 435 GWh over the same period (see: Fig. 6).
NC has very good solar potential which should enable up to 100 % of thermal energy demand of a building to be met. Therefore the importance of the link (and mismatch) between solar energy and the heat load of a building should be understood and innovative methods to store heat with THS materials developed. This will start a transformation in NC towards a ‘green’ island and a cleaner and safer environment to live.
Materials and methodology
To evaluate the technical, economic and environmental feasibility of ‘open’ THS systems under NC climate conditions, initially, the annual heat load of a real residential building was simulated. The annual solar heat gain was obtained assuming a total collector surface area, Acoll, = 8 m2. The THS material was selected as SIM-3a (i.e. Vermiculite impregnated with CaCl2) from previous research carried out by the authors (Casey et al. 2014). This absorbent composite has many remarkable properties such as a high experimental heat storage density, Ed = 165 kWh/m3, good cyclic ability with little hysteresis, low regeneration temperature, Tr < 90 °C and is non-toxic. The storage density of SIM-3a is used to calculate the overall dimensions of the proposed TES system.
A schematic diagram of the proposed THS system is shown in Fig. 7. The system is comprised of solar air collectors (1), an evaporative humidifier (2), an over or underground THS unit (3) and internal heat outlets (4) in the building (5). In the summer period (i.e. June to August), the system operates in charging mode, as an open cycle, where the solar collectors supply hot and dry air to the THS (Tr = 80 → 90 °C). The hot air charges the system by heating the THS material and removing any moisture. The outlet air from the THS during charging is moist and warm (Tout = 50 → 55 °C). Although there is not any heating demand in summer conditions, there is a requirement for hot water, particularly after sundown. Therefore any heat from the outlet air can be transferred to a hot water tank (6) via a heat exchanger before dumping waste heat to the ambient air.
During the winter (i.e. December to February), the available solar energy is not sufficient to meet the total heating demand of the building during the day. In addition after sundown when heating demand peaks, solar energy is not available. In this instance (i.e. discharging mode) the THS is used to supplement any heat gained from the collectors providing adequate heat to the building. Placing the store underground would also minimize heat loss to the surroundings enabling longer heat generation or dwell times, tdwell from the THS whilst also minimizing space usage inside or around the building. In this setup, the system operates in ‘closed’ cycle mode to minimize any waste heat (see: Fig. 8). As an evaporative humidifier is used to introduce moisture to the input airstream, heat pipes can be used to gain some heat from the ground to ease evaporation of water in the humidifying tank. The relative humidity (RH) of the input air is increased to RH = 80 % and blown to the THS.
The THS system consists of divided units with perforated diffusion tubes in each unit as schematically described in Figs. 7 and 8. The perforated tubes enable air to diffuse throughout the absorbent uniformly which is crucial for the maximising reaction performance. The tubes overcome the issue of increased air resistance at the front surface of the material due to the thickness of the absorbent bed. This resistance can slow down the reaction kinetics whilst also causing a pressure drop which results in high fan energy consumption and low temperature lift. Similarly to the diffusion tubes, perforated collector pipes are connected to the outlet side of the reactor. This allows the humid air to diffuse through the material and the hot air to be collected via outlet pipes. This concept increases the contact area of air and absorbent providing one possible solution to non-uniform HAM transfer in larger scale applications.
Building thermal analysis
The chosen dwelling is a 40 year old, single floor building with a floor area, A = 180 m2 (see: Fig. 9). The building is located near the west coast of the island where the weather is hot and humid in summer and mild in spring and autumn. Although Cyprus does not suffer extremely severe winter conditions, ambient temperature is below comfort conditions (18 → 20 °C) during most of the winter, and top-up heating is required. The chosen dwelling uses three types of heating systems to provide this;
- Radiative electrical heaters are installed in the bedrooms
- A gas heater is used in kitchen and one of the living rooms
- Air conditioning units are used for the remaining rooms
To simulate the heat load of the building the software package Ecotect Analysis 2011 was used. The software allows user to create annual simulations using a 3D model of the building with imported building properties (i.e. dimensions, U-Values, ventilation etc.). The software also provides the user with several adjustable settings. For this work the heat gain from occupants was set at 70 W (i.e. 4 adults living in the building), the air conditioning type was set on 24 h heating mode whilst sensible and latent heat gains were set at 5 W/m2 and 2 W/m2 respectively. The 3D model created in Ecotect, using the original dimensions of the building is illustrated in Fig. 10.
In NC, insulation of external building envelope elements is not considered important and is rarely utilised, whilst winter heat load is not as high as other northern countries such as UK, Norway or Finland, increased application of insulation could provide huge monetary and energy savings. The building case here is primarily of a concrete type construction with uninsulated brick masonry walls however, the roof is minimally insulated using an asphalt coating to the exterior. The buildings windows are of single glazed construction with an unbroken aluminium frame. The properties of the building components with their applicable surface areas are presented in Table 1 and the data used in Ecotect to perform the thermal analysis. Comfort conditions for the building were chosen as 18 °C for the heat load analysis.
|Building component||Material||Total surface area (m2)||Thickness (mm)||Schematic view||U-Value (W/m2K)||Admittance (W/m2K)||Solar absorption coefficient|
|Wall||Brick plaster||138.3||Brick: 110||2.62||4.38||0.42|
|Door||Solid oak timber||7.31||40||2.26||3.19||0.46|
|Window||Single glazed- aluminium frame||38.8||6||6.0||6.0||0.94|
|Roof||Concrete roof-Asphalt||185.5||Asphalt: 6||0.89||2.30||0.90|
|Floor||Concrete slab-on ground||185.5||Soil:1500||0.88||6.0||0.47|
The annual total solar radiation, It on an optimally inclined solar collector, hours of sunshine, hs and average ambient temperature, Ta, are presented in Fig. 11. The comfort temperature, Tc used for analysing the heat load of the building, was set as 18 °C. As can be seen year round solar irradiation is in the range of 4 → 7.5 kWh, ambient temperature is 12 → 28 °C and daily sunshine is in the range of 9.5 → 14 h. According to these data, it is clear that Cyprus has a remarkable year round solar availability to utilize in thermal applications. Data obtained from the European Commission’s web site (http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php?lang=en&map=europe. [Accessed on 10.01.2015], http://photovoltaic-software.com/pvgis.php) was used to calculate the solar thermal heat gain, Qu (W) using Eq. 1:
Where Ac is the collector area (m2), Ht is the solar radiation (J/m2), hs is the radiation time (s), τα is the absorption-transmission coefficient of solar collectors, UL is the heat loss coefficient of solar collectors (W/m2 °C), Tfi is the collector surface temperature (°C) and Ta is ambient temperature (°C). The properties of the solar collectors that were used in the calculation are given in Table 2.
|Total collector area, Ac||m2||8|
|Collector heat exchanger efficiency, Fr||–||0.86|
|Absorption-Transmission coefficient, τα||–||0.8|
|Heat loss coefficient, UL||W/m2 °C||6.9|
A survey was conducted by Evcil (Evcil 2012) to demonstrate the general properties of dwellings in NC with the results given in Table 3. The dwelling for this study was chosen as its properties are representative of the average dwelling in NC as described in the Evcil study. The survey consisted of multiple choice questions and results demonstrate the share of different options provided in questions. The properties of the building considered in this study are highlighted in the table.
|Percentage of total, %|
|Number of occupants||1||2||3||4||>5|
|1.7 %||13.9 %||27.7 %||45.4 %||11.3 %|
|Type of houses||Detached||Semi detached||Flat||–||–|
|52.6 %||5.3 %||42.1 %||–||–|
|Age of houses||<5 years||5–10 years||10–20 years||>20 years||–|
|10.4 %||20.8 %||38.1 %||30.7 %||–|
|Area of houses||<100 m2||100–200 m2||>200 m2||–||–|
|10.8 %||81.8 %||7.4 %||–||–|
|Type of external walls||Terracotta brick||Concrete blocks||Others||–||–|
|91.3 %||8.4 %||0.3 %||–||–|
|Insulation-external walls||Glass wool||Polystyrene||Air gap||No insulation||–|
|1.3 %||3.5 %||1.7 %||93.5 %||–|
|Frames of windows||Wood||Aluminium||PVC||–||–|
|16.9 %||78.4 %||4.7 %||–||–|
|Glazing||Single glazing||Double glazing||–||–||–|
|84.8 %||15.2 %||–||–||–|
|Type of roof||Flat Concrete||Inclined concrete w tiles||Flat concrete, tiles on wood||–||–|
|73.6 %||8.7 %||17.7 %||–||–|
|Insulation-roof||Glass wool||Polystyrene||No insulation||Asphalt||–|
|4.4 %||7.1 %||88.5 %||–||–|
|Means of heating||Central heating||Heaters||Air-conditioners||–||–|
|2.2 %||53.0 %||44.8 %||–||–|
|Energy source for heating||Wood||Electricity||LPG||Kerosene||Diesel|
|1.3 %||65.8 %||27.7 %||3.9 %||1.3 %|
|Number of air conditioners||None||1||2||3||>4|
|30.7 %||21.2 %||23.8 %||16.5 %||7.8 %|
The main aim of the analysis was to determine the storage volume, Vs of THS required to assist the buildings normal solar gains and fully meet building heat demand. As a ‘seasonal’ heat storage approach is proposed, the potential of solar energy for absorbent regeneration was also investigated. The absorbent material selected was the SIM-3a (V-CaCl2) composite as discussed in previous section with the Ed used in order to determine Vs.
In the dual hybrid model proposed here, integrated THS and solar air collectors are considered as the heating unit. In Fig. 12 the annual heat load and solar heat gain for NC climate conditions is presented. There is no heat load from May to October as the average temperature is higher than 18 °C (i.e. chosen comfort limit) during this period. The heating period begins in November and lasts until April.
However in November, March and April solar heat gains still exceed the building heat demand, Qu > Qh therefore there is no necessity for any THS in these months. THS is only required in December, January and February where heat loads, Qh are greater than heat gains are (i.e. Qh = 883, 1310 and 1060 kWh whilst Qu = 674, 582 and 683 for Dec, Jan and Feb respectively).
These results indicate that an auxiliary heating source is required to assist solar energy for full compensation of heat load of the building. The peak difference, ∆Q between Qu and Qh occurs in January with 729 kWh with February and December having ∆Q of 376.2 kWh and 209.3 kWh (see: Fig. 13). The total additional thermal energy required for the three heating months is 1314 kWh and, based on the energy density of the absorbent (Ed = 165 kWh/m3), the THS volume required to meet the total thermal demand was calculated as Vs = 8 m3. The specific required storage volume for each month is calculated as 4.41, 2.28 and 1.26 m3 in January, February and December respectively.
From May onwards, when there is no building heat load and the moist absorbent can be regenerated using solar energy. From the second half of spring to the first half of autumn (April–September) solar radiation is at its maximum level (> 6 kWh/m2) and the notional 8 m3 absorbent can be regenerated in a time period of slightly more than one month as presented in Fig. 14. The theoretical amount of energy required for regeneration is equal to the energy supplied by the material during the discharging process (i.e. Qc = 1314 kWh = Qd) and is indicated in Fig. 14. From May to October, the potential cumulative solar gain is 7582 kWh and thus only 17 % of this would be sufficient for regenerating the 8 m3 of SIM-3a. Any remaining heat can be used for water heating purposes, if required.
CO2 savings and economic evaluation of the system
Control of GHG and CO2 emissions and methods for reducing them have been gaining importance in recent years as they have been shown to have a significant effect on both Ozone depletion and global warming. Utilizing new, clean and renewable energy sources is one of the most promising solutions to this problem. In the domestic sector of NC, electricity, natural gas, fuel oil, LPG or solid fuel sourced systems are mostly preferred for space heating purposes due to their advantages for consumers such as lower prices, steady and easy operating conditions, low space requirements and long operational lifetime in comparison with renewable sourced heating systems.
Space heating applications however do not require high temperature. For example the heat pump blowing temperature is 35 → 45 °C in air conditioning applications or water circulation temperature is 35 → 40 °C for wall or ground heating methods. Using high temperature combusting fossil fuels for low temperature applications like space heating is a waste of the high quantity and quality of this energy (i.e. as a result of exergy destruction). In addition this is the waste of the fuel potential, which could be used for more appropriate applications such as electricity generation. The potential contribution of THS systems would facilitate this move away from high temperature fuel wastage by meeting the space heating demand with solar energy.
The conversion factor, ς is a coefficient used to determine the CO2 emissions from energy generation or consumption processes and varies in the range of 0 → 1 depending on several factors (e.g. fuel type used etc.). The conversion factor of electricity per kWh consumption, ςe, in Cyprus is 0.87 whilst for LPG, including residual heat or power generation ςLPG = 0.21. According to a recent IEA report however, solar thermal and THS systems do not have CO2 emissions and therefore.
Based on these calculations, utilizing solar assisted THS for space heating would allow for a saving of 3.65, 1,21 and 0.88 ton CO2 per year compared with usage of EH, GH and HP respectively (see: Fig. 15). The building investigated here uses all of these systems. The living room and one of the bedrooms are heated using a HP, the kitchen and other living room are heated with GH and rest of the rooms are heated with EH. For this reason CO2 savings for the investigated building are determined as an average of the CO2 savings achieved for all three heating methods. Therefore yearly CO2 savings of the investigated building with the usage of solar assisted THS, is estimated at 1.91 ton per year. As the system lifetime is assumed to be 25 years and during that period savings of 47.9 ton CO2 are expected.
In addition to the environmental effect of the proposed system, the economical evaluation of it should also be discussed in order to form a complete picture of solar assisted THS systems. The total cost of the system is estimated at €2750 based on the information provided in Table 4. Cost estimations are made based on component and material retail prices in NC at the time of printing.
|System component||Price (€)|
|Ducting pipes, fittings, vents||200|
|Solar panels (8 m2)||600|
|Heat storage tank (8 m3)||400|
|Heat storage material (8 m3)||800|
|System built and installation costs||500|
For analysis of the total savings of the proposed system, the present worth method is used. According to this method The rate of interest, r, can be found using Equation 5:
The potential savings against using the HP, EH and GH over the 25 year period are analysed using Equations 7, 8 and 9 respectively where is the total operational cost, ξ the total yearly energy consumption and fe, the unit price of electricity in NC which is currently €0.22/kWh for domestic usage.
The total cost of the THS, 10 where subscripts O, M and C represent operational, maintenance and capital respectively.THS, through the 25 year period including capital cost, operational and maintenance costs is calculated using Equation
The total yearly operational and maintenance costs of the system are expected to be very low with the annual sum assumed as 2 % of the total capital costs (see: Equation 11);
The payback periods of HP, EH and GH are determined as follows;
The economic analysis results are presented in Fig. 16. Similar to the environmental analysis, this is conducted based on the savings of solar assisted THS compared with the operating costs of usage of the current systems/methods (HP, EH and GH) and the average. It was found that the solar assisted THS system could provide a saving of €550/year whilst cumulative savings exceed the sum of total (capital, maintenance and operating) cost of system, (3081 > €3050) at the end of year 6 years. Over the full 25 years the cumulative total saving would be €10,077 whereas the total cumulative cost of the system would be €3731 over the same period corresponding to a €6346 profit with the usage of THS compared to HP, GH and EH. The average annual energy consumption (e.g. appliances, heating cooling, hot water) of the investigated building is ~5600 kWh whilst the energy (electricity, gas) consumed for space heating is ~ 1900 kWh. Therefore the average annual energy saving rate is found to be 34 %. The floor area of the building investigated is 180 m2 whereas the floor area of dwellings in NC is generally 100–150 m2 and therefore, in NC conditions the average annual energy saving rate and payback period is expected to be somewhat less than the investigated building i.e. 25–30 % and 5–7 years respectively.
In this study the applicability of thermochemical heat storage (THS) under Northern Cyprus climate conditions is theoretically investigated. An underground THS system, integrated with 8 m2 solar air collectors placed on the roof of a flat roof, detached building is considered. In addition, an evaporative humidification unit for humidifying the air during the discharging process is assumed. It was found that 8 m3 of absorbent was required to meet the mismatch between solar energy supply and building heat load where the total solar heat gain and heat load are 1939 kWh and 3253 kWh respectively. On the other hand with 8 m2 of solar air collectors, 35 days in the summer period (May/June) where the solar irradiation is in the range of 6 → 7 kWh/m2 are required to regenerate the absorbent.
Gas heaters, electrical heaters and heat pumps are widely used in North Cyprus for space heating. The results of an economic and environmental comparison showed that payback time would be 6 years based on the capital and operational costs of THS compared to the average operational costs of GH, EH and HP. Similarly, CO2 savings would be 47.9 tonnes.
According to the theoretical analysis, THS was found to be a feasible method to utilize for space heating purposes in Northern Cyprus. It has very good potential to replace electrical and gas sourced heating systems which have negative effects on the environment and economy. The remarkable solar potential in Northern Cyprus, which is > 6 kWh in summer should be harvested to meet the heating demand in winter by utilizing THS. In this respect, the Northern Cyprus government should raise the public awareness and promote a transformation away from traditional space heating systems for a sustainable future.
|fe||electricity unit price||€/kWh|
|Fr||collector heat exchanger efficiency||–|
|Ht||total solar irradiation||J/m2|
|It||total solar irradiation||kWh/m2|
|Qu||solar heat gain||kWh|
|Qh||heat load of the building||kWh|
|r||rate of interest||–|
|UL||collector heat loss coefficient||W/m2K|
|conversion factor of electricity per kWh consumption||–|
|ξ||total yearly energy consumption||kWh|
|Υ||total operational cost||€/kWh|
|c||comfort, collector, charging, capital cost|