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The relation of collector and storage tank size in solar heating systems

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DOI: 10.1016/j.enconman.2012.01.031

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 Energy Conversion and Management 63 (2012) 112–117

Contents lists available at SciVerse ScienceDirect

Energy Conversion and Management

journal homepage: www.elsevier.com/locate/enconman

The relation of collector and storage tank size in solar heating systems
Kemal Çomaklı a,⇑, Uğur Çakır b, Mehmet Kaya c, Kadir Bakirci a
a
Department of Mechanical Engineering, Atatürk University, Erzurum, Turkey
b
Department of Mechanical Engineering, Bayburt University, Bayburt, Turkey
c
Department of Mechanical Engineering, Erzincan University, Erzincan, Turkey

a r t i c l e i n f o a b s t r a c t

Article history: The most popular method to benefit from the solar energy is to use solar water heating systems since it is
Available online 7 April 2012 one of the cheapest way to benefit from the solar energy. The investment cost of a solar water heating
system is very low, and the maintenance costs are nearly zero. Using the solar energy for solar water
Keywords: heating (SWH) technology has been greatly improved during the past century. A storage tank is used
Solar energy in many solar water heating systems for the conservation of heat energy or hot water for use when some
Solar heating systems need it. In addition, domestic hot water consumption is strongly variable in many buildings. It depends
Storage tank
on the geographical situation, also on the country customs, and of course on the type of building usage.
Above all, it depends on the inhabitants’ specific lifestyle. For that reason, to provide the hot water for
consumption at the desirable temperature whenever inhabitants require it, there must be a good rele-
vance between the collectors and storage tank. In this paper, the optimum sizes of the collectors and
the storage tank are determined to design more economic and efficient solar water heating systems. A
program has been developed and validated with the experimental study and environmental data. The
environmental data were obtained through a whole year of operation for Erzurum, Turkey.
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction tion has expanded significantly in recent years [8]. Using solar en-
ergy systems have a good potential for production of heat energy
In the future the world’s energy supply must become more sus- to consume it for domestic purposes like residential space heating
tainable. This can be achieved by a more efficient use of energy and or producing domestic hot water. Energy consumption of a typical
relying on renewable sources of energy, particularly wind, hydro- family which lives in an ordinary house is less than the solar radi-
power, solar and geothermal energy [1]. The studies on the new ation energy that reaches to the roof of the home from the sun. The
and renewable energy sources have gained speed and are encour- relatively low temperatures required for heating and domestic hot
aged because energy resources used today are rapidly depleted and water applications make solar collection efficiency relatively high:
cause environmental pollution [2]. Solar energy arriving on earth is for example, according to Cruickshank [9], thermal systems are
the most fundamental renewable energy source in nature. Solar typically two to four times as efficient as photovoltaic (PV) sys-
energy occupies one of the most important places among various tems. Furthermore, space and water heating are responsible for a
alternative energy sources [3]. Solar energy technologies offer a large portion of the energy needs of residential buildings: about
clean, renewable and domestic energy source, and are essential 82% in Turkey and 82 in Europe [10]. That means, there is a big po-
components of a sustainable energy future [4]. The systems of solar tential in using solar thermal technologies to convert solar radia-
water heating and solar source heat pump provide a new and clean tion in to usable sensible heat. Nevertheless; consumption of
way of heating buildings in the world. Therefore, these systems can fossil fuels must diminish in order to reduce CO2, SOx, and NOx
be used to minimize environmental impacts and air emission [5]. emissions to the atmosphere; moreover, this energy source is
They offer the most energy-efficient way to provide heating and being limited by factors such as natural source depletion, environ-
cooling in many applications, as they can use renewable heat mental damage and economics. Because of the reasons mentioned
sources in our surroundings [6,7]. above many governments have decided to strengthen their na-
Water heating using domestic solar water heaters is the most tional efforts to increase the deployment of energy conservation
feasible, economical and popular means of solar energy utilization technologies and increase utilization of renewable energy sources
in many countries in the world. The world market for their utiliza- especially solar energy. However the solar energy source is only
a small contribution to the total energy demand, for several rea-
⇑ Corresponding author. Tel.: +90 442 231 48 49; fax: +90 442 236 09 57. sons varying from cost effectiveness to long-term technological
E-mail addresses: kcomakli@atauni.edu.tr, kcomakli@hotmail.com (K. Çomaklı). reliability. Therefore further attempts are being made to resolve

0196-8904/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.enconman.2012.01.031
 K. Çomaklı et al. / Energy Conversion and Management 63 (2012) 112–117 113

these issues like as energy storage systems. Heat storage for solar According to Braun et al. [19], significant reductions in solar col-
thermal systems is a way to compensate the mismatch between lector requirements for heating could be achieved by using sea-
heat production and energy needs [11]. sonal storage at northern latitudes, where seasonal variations are
The thermal energy storage is not only plays an important role large, and in cold climates, where domestic hot water loads are
in conservation the energy but also effects the performance and much smaller than space heating loads. Similarly, Hooper [21] sta-
reliability of wide range energy systems, and become more impor- ted that, a solar heating system with a 100% solar fraction1 for a
tant where the energy source is intermittent such as solar. The Canadian home using seasonal storage would require 25% of the
thermal energy storage can be used in places where there is varia- collector surface needed for the same system type equipped with
tion in solar energy or in areas there is a high difference of temper- short-term storage. Since solar collectors tend to be expensive,
ature between day and night [12]. there is definitely potential in developing more economical storage
Solar heating systems, especially solar domestic heating water systems in order to obtain higher solar fractions for these heating
system, consist of solar collectors, thermal reservoir, auxiliary tasks. The solar fraction is the ratio of the heat used for space heat-
heater, heat exchanger, and a series of junction tubes which con- ing and DHW production coming from the solar heating system to
nect solar collector into storage tank. The majority of the domes- the total heat requirements for these tasks.
tic solar water heater systems use flat plate collectors, which are Sillman [22] evaluated that the performance of systems
connected horizontal or vertical storage tanks. The water inside equipped with seasonal storage increases linearly as the storage
the storage tank is heated through liquid circulation in the collec- size increases up to the point of unconstrained operation, i.e., the
tor cycle. Collector cycles are designed as open (direct) or closed point where the storage size is sufficient to store all the heat col-
(closed) circuits. In an open circuit cycle, fluid inside the collector lected during summer. He concluded that this point of uncon-
directly poured into the storage tank. In the case of closed circuit strained operation is the likely economic optimum. He also
cycle, in order to transform the heat energy between collector evaluated that systems with seasonal storage providing nearly
and water inside the tank, a heat exchanger is used. One of the 100% solar space heating may cost the same or less per unit heat
most common types of the heat exchanger used in domestic solar delivered than systems equipped with diurnal storage providing
water heaters is a circular shell surrounding the peripheral of the only 50% of the space heating.
tank and is called mantle heat exchanger. In these systems collec- In the case of annual performance evaluations, however, it is a
tor outlet is connected between the middle and upper part of the standard practice to use simplified computer algorithms to reduce
heat exchanger. The usable hot water is extracted from upper computational over-head and computing times. The complexity of
part of the storage tank and is replaced by cold city water both component and system models is often weighed against ‘‘user
[13,14]. The heat storage tank is a key part of the solar water convenience’’, computing time and resources, and desired accu-
heating systems which stores thermal energy in the form of hot racy. In many instances, due to the lack of detailed information, a
water during the day-time and delivers it to the user at the night number of simplifying assumptions are usually made in the model.
time or demand. The success of this process relies on the accurate specification of
There are many experimental and theoretical–numerical stud- the system’s physical and thermal characteristics and the complex-
ies made on the performance evaluation of solar heating systems ity and underlying assumptions of the computer algorithm [23].
and storage tanks used on them under different operation or de- In many solar water heating or solar space heating applications,
sign parameters in the literature [8–23]. These parameters primar- thermal storage tanks are left to be larger than it is needed. Using
ily include geometrical conditions; using horizontal or vertical larger or excessive storage tanks on solar heating systems decrease
storage, the ratio of the tank height to diameter, wall thickness, the efficiency and increases the cost of the system. Therefore, this
wall substance, the size of tank inlet band outlet openings, opera- study investigates the relation between the total area of the solar
tional conditions, i.e., inlet fluid speed, the difference between inlet collectors and the volume of the storage tank which is used for
and outlet fluid temperatures stored in the tank and climatic accumulating the thermal energy gained from the sun by the col-
conditions. lectors. To achieve this aim, a numerical program in MATLAB was
An experimental study was made by Lavan and Thompson [15] developed with the help of the data obtained from the experiments
on a thermally stratified, vertical hot water storage tank. The performed under the climatic conditions of Erzurum, Turkey.
experiments were conducted for various height/diameter ratios,
inlet port location and geometry, inlet–outlet temperature differ-
ence and different mass flow rate. Their study showed that better 2. Solar heating collectors and storage tanks
thermal stratification can be obtained by increasing the ratio of
the tank height to its diameter, increasing the diameter of the inlet Hot water consumption is strongly variable especially in the
port, and or increasing the difference between the inlet and outlet residential buildings. Some of the parameters which affect the
water temperature. Cole and Bellinger [16] suggested that by hot water consumption can be given as geographical situation,
selecting the ratio of height to diameter of the storage tank equal country customs, type of building usage and general life styles of
to four, maximum thermal stratification in the tank can be the people in the country. Above all, the most important parameter
achieved. The study of Ismail et al. [17] confirmed Cole and Bellin- is the specific lifestyles of the inhabitants [24,25]. All these factors
ger conclusion while Nelson et al. [18] suggested the optimum va- lead to the need of storing the hot water in tanks in order to have
lue of 3 for the tank height to diameter ratio. water prepared for consumption at the desired temperature when-
In addition, size of the storage tank or the ratio of the volume of ever inhabitants require it. When the domestic hot water is heated
the storage tank and total area of the solar collectors are very by a solar thermal plant, storage is mandatory in residential build-
important parameters for designing economic and efficient solar ings. For that reason, to determine the relation between the profile
water heating systems. Braun et al. [19] evaluated that storage of daily solar irradiance and profile of hot water consumption is
capacities per unit of collector area must be two to three orders very important to achieve more efficient solar water heating
of magnitude (100–1000 times) larger for seasonal storage than systems.
for overnight storage. Nevertheless, Fish et al. [20] reported invest- Design parameters such as collector direction or position and
ment costs per square meter of solar collector for large scale solar the size of the components, which are used on solar water heating
plants only twice as high for systems with seasonal storage than systems like storage tank or collector area, depend on the meteoro-
for systems with short-term storage. logical specifications (i.e., radiation intensity, air temperature, tap
114 K. Çomaklı et al. / Energy Conversion and Management 63 (2012) 112–117

water temperature) of the region in which the system will be set and TS 3817 standards (Turkish Standards Organization). The stor-
up on and the thermal energy demand for hot water. The thermal age tank was insulated well to eliminate heat losses and collectors
energy demand and utilized radiation energy vary according to the placed on the roof with the slope angle of 30°. The total collectors
time of the year and time of the day. Meanwhile, the energy that surface area is 3.8 m2 and the glass covers were cleaned in the
obtained from the unit are of collector does not only vary according morning. A heat exchanger is used to transfer the heat energy from
to the specifications of the collector. It also varies according to the the heated fluid in the collectors to the water used by end users in
system components’ sizes and properties. the system. Two water pumps are used to circulate the fluid in the
Collectors and storage tanks are the most important components collectors to the heat exchanger, and the fluid in the storage tank to
of the solar heating systems. Usually the collectors used on solar the heat exchanger.
water heating systems are the most expensive members of the solar Fig. 1 is presented to shows the experimental system schemat-
systems. The collector types mostly used on solar water heating sys- ically. As seen in Fig. 1, the relevant inlet and outlet temperatures
tems are flat plate, high efficiency-flat plate, vacuum tubes and heat of each component of the system and the temperature of the stor-
pipe collectors. Commercial water tanks can be divided into two age tank from three different points of the tank were measured by
types in general, depending on the existence of serpentine tubes in- using thermocouples and recorded. In addition the mass flow rates
side them or not. Apart from the previous classification, they can be of the water which circulates in different parts of the system were
classified depending on the material they are made of, such as stain- measured using flow meters and evaluated. The solar radiation
less steel, vitrified steel and steel with an epoxy cover, all of them coming onto the unit area of horizontal surface was determined
being suitable for contact with potable water. and recorded by using a pyranometer. All of the instrumentations
In order to meet the thermal energy and hot water demand of used on the system were calibrated once installed in the plant in
the buildings regularly, it is needed to determine the volume of order to reduce the possible measurement errors.
the hot water storage tank and the total area of the collector. Net
absorber area of the collector could be determined by making 3.2. Climate properties
use of the local solar radiation data of the related region. If time
and rate of energy generated by a solar energy system do not coin- The experimental solar heating system was established and
cide with energy needs, then thermal energy storage is required to tested in Erzurum. The annual heating degree days for Erzurum
store the excess generated energy until it is needed. Ideally, the with a base temperature of 18 °C is 4870 [26]. The energy require-
storage capacity should be sufficient to store any excess energy ment for the months of the heating season is maximum in January
at a temperature most beneficial to both energy collection and en- (with 889° days), while it is minimum in May (with 236° days).
ergy usage. The size of the storage tank could be approximated Yearly total Nbin values for six time periods in the province of
according to the specification of the load pattern as well as the esti- Erzurum in the region of the East Anatolia of Turkey were calcu-
mated average heating power output of the solar heating system. lated [27]. The hours of the smallest temperature bin of 19.5 °C
The economic aspect should be also considered for the selection observed in Erzurum are 17 in both January and February. Also,
of the tank size. the hours of the temperature bin from 21 °C to 18 °C for Erzurum
were computed as 7498 in the average of 1995–2005.
3. Liquid-based solar water heating system and its Some of the meteorological and climatically data of Erzurum are
mathematical formulation shown in Table 1 and Fig. 2, where n is the day number of related
month, in which day means the day of the month which represents
3.1. Experimental solar water heating system the general-average climatic values of the month, Ttap is the tem-
perature of the tap water, TL, TAV and TH are the lowest, average
We introduced a liquid based domestic water heating systems and highest air temperature values of the day, t is the duration of
with antifreeze solution collector loop as shown in Fig. 1, which solar radiation for a day, Q is the monthly average daily global radi-
is a typical heating system recommended for use in cold climates. ation reaching to the unit area of horizontal surface and Qo is the
The modeling study was performed considering a 10 yr old simple monthly average daily extraterrestrial radiation coming to the unit
experimental solar energy system with two collectors in Erzurum, tilted area of the collectors.
which is one of the coldest cities in Turkey, in the cold climate re-
gion on north-east area of Turkey. While the system had been de- 3.3. Methodology and model description
signed, the size of the equipments was determined considering the
factors such as the system that is used by a family with four mem- We proceed to analysis the particular systems of Fig. 1 by pre-
bers in a household and heated water is stored in a storage tank. senting to appropriate equations governing its various
The volume of the tank was determined as 220 L by using TS 825 components.

Expansion Tw,o
Tank

Domestic
Ht To Hot
Ts Water
Storage Tank

Pyranometer
Ts,i Qw
Qc
Tw,i
Exchanger

Qs
Heat

Ti

Flow meter
Cold
Ts,o Water
Water Pump

Fig. 1. Experimental solar water heating system.

 K. Çomaklı et al. / Energy Conversion and Management 63 (2012) 112–117 115

Table 1
Some meteorological and climatically data of Erzurum.

Month n Day Ttap (°C) Tair (°C) t (h) Q (MJ/m2 day) Qo (MJ/m2 day)
TL TAV TH
January 31 17 3.29 16.9 10.8 4.4 2.95 8.66 15.3
February 28 16 1.66 16.7 10.1 3.1 3.82 12.58 20.55
March 31 16 1.54 9.8 3.7 2.6 4.39 15.97 27.4
April 30 15 3.88 0.9 5.2 11.8 5.89 16.95 34.4
May 31 15 8.49 2.7 10.3 17.3 7.56 19.89 39.4
June 30 11 12.81 5.5 14.6 22.3 9.92 23.17 41.32
July 31 17 16.49 9.6 19.1 27.2 10.75 23.19 40.26
August 31 16 18.51 9.1 18.9 27.7 10.54 21.21 36.23
September 30 15 17.69 3.5 13.6 23.3 8.31 17.25 29.83
October 31 15 14.48 0.6 7.4 16.3 6.28 12.57 22.52
November 30 14 10.04 6.8 0.5 6.9 4.26 8.79 16.52
December 31 10 5.99 12.6 7.2 1.6 2.31 7.03 13.82

Q_ s ¼ ðmc
_ p Þs ðT s;i  T s;o Þ ¼ eðmC
_ p Þc ðT o  T i Þ ð2Þ
where e is the heat exchanger effectiveness, ðmc _ p Þs is the storage
tank the mass flow rate- specific heat capacity of water, Ts,i and
Ts,o are the temperature at the inlet and outlet of storage tank,
respectively. By neglecting thermal stratification in the storage
tank, the rate of internal energy change of the tank is given by
dT s
_ pÞ
ðmc ¼ Q_ s  Q_ w  Q_ TL ð3Þ
dt
where ðmc _ p Þ is the mass and specific heat product of water in the
storage tank, t is the time, Qw,s is the service water heating load sup-
plied by solar energy via the service water heat exchanger and QTL is
the rate of energy loss from the storage tank. The rate of tank energy
Fig. 2. Variation of the thermal demand and solar radiation according to the loss, QTL, is given by the expression
months for Erzurum.
Q_ TL ¼ ðUAÞs ðT s  T e Þ ð4Þ
where (UA)s is the storage tank loss coefficient and area product.
The heat energy transferred to the working fluid in the pipe of a
The service water heating load supplied by solar energy via the ser-
collector can be written as:
vice water heat exchanger, Qw, can be expressed as
Q_ c ¼ ðmC
_ p Þc ðT o  T i Þ ð1Þ Q_ w ¼ ðmc
_ p Þw ðT w;o  T w;i Þ ð5Þ

where ðmC_ p Þc is the collector the mass flow rate- specific heat An energetic efficiency equation for the system is written as:
capacity of fluid, Ti and To are the temperature of fluid at the inlet
Q_ s
and outlet of the solar collectors, respectively. By neglecting piping gs ¼ ð6Þ
Ac Ht
heat loss, the energy that transferred by the heat exchanger, from
working fluid to the usable water stored in the tank in closed sys- where Ac is total surface area of collectors, and Ht is the solar radi-
tem can be defined as follow, ation energy that reaches to the per unit area of collector.

Fig. 3. Change of collector efficiency according to the rate of the storage tank volume to the collector area.
116 K. Çomaklı et al. / Energy Conversion and Management 63 (2012) 112–117

Fig. 4. Change of usable water temperature according to the rate of the storage tank volume to the collector area.

Fig. 5. Change of collected energy by system according to the rate of the storage tank volume to the collector area.

4. Results and discussion Generally, the temperature of the usable water is required to be
in range of 40–70 °C in Turkey according to the related standards
Before starting this study, the transient model of the compo- and practical assuming. As seen in Figures although the efficiency
nents and the model for whole system were validated experimen- of collector increases, depending on the increase of the volume of
tally, which all of them have been integrated in a unique program. the storage tank, the temperature of the usable hot water de-
After the validation study some modifications and improvements creases. It is very important to provide the usable hot water to
were made on the model. the users at enough temperature level as they need. Additionally,
Fig. 3 shows that when the volume of the storage tank in- as can be understood from Fig. 3, increasing rate of the collector
creases, the efficiency of the collector increases, According to efficiency goes down when the rate of the tank volume-collector
Fig. 3, when the tank volume to collector area is 200 L/m2, the effi- area goes up to the value of 200 L/m2 or higher. Due to the reasons
ciency of the collector becomes the highest while it becomes the mentioned previously, determining the optimum tank size accord-
lowest when that ratio is 25 L/m2. ing to the collector area is very important for designing more use-
Fig. 4 shows the relation between the average temperature va- ful and more beneficial and more economic solar heating systems.
lue of the water in the tank and tank volume ratio to collector area.
As seen in the figure, the temperature of the storage tank de- 5. Conclusion
creases, while the volume of the tank increases. When the vol-
ume-tank ratio has the lowest value of 25 L/m2, the annual value This study presented reveals that the size of the storage tanks
of the temperature of the tank becomes the highest. It can be seen affects the performance and usability of solar water heating sys-
from Fig. 5 that, as seen in Fig. 3, the amount of the collected ther- tems. In practical applications, due to the lack of some physical
mal energy increases when the volume of the storage tank accuracy and basic rules, thermal storage tanks are sometimes left
increases. to be as large as the designer decides. In addition to that a larger
 K. Çomaklı et al. / Energy Conversion and Management 63 (2012) 112–117 117

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