ECET 521: Chapter Summaries and Assignments
Chapter 5: Water Heating Systems
“The most popular application of solar systems is for domestic water heating…[because]
relatively simple systems are involved and solar water heating systems are generally viable…A solar
water heater is a combination of a solar collector array, an energy transfer system, and a storage tank. The
main part of a solar water heater is the solar collector array, which absorbs solar radiation and converts it
to heat. This heat is then absorbed by a heat transfer fluid (water, non-freezing liquid, or air) that passes
through the collector. This heat can then be stored or used directly. Because it is understood that portions
of the solar energy system are exposed to weather conditions, they must be protected from freezing and
overheating caused by high insolation levels during periods of low energy demand.
Two types of solar water of solar water heating systems are available:

Direct or open loop systems, in which potable water is heated directly in the collector.

Indirect or closed loop systems, in which potable water is heated indirectly by a heat
transfer fluid that is heated in the collector and passes through a heat exchanger to
transfer its heat to the domestic or service water.
Systems differ also with respect to the way the heat transfer fluid is transported:

Natural (or passive systems) [which use natural convection to circulate the fluid]

Forced circulation (or active)systems [which use fans and pumps to circulate the fluid]”
(Kalogirou, 2009, p. 251)
These systems are further differentiated as follows:
Types
Passive Systems
Active Systems
Thermosiphon (direct and
indirect)
Direct circulation (or open loop
active) systems
Integrated collector storage
Indirect circulation (or closed
loop active) systems
Internal and external heat
exchanger
Air systems
Heat pump systems
Pool heating systems
Control
No control
Thermostats
Freezer Protection
Drain-back
Recirculation and drain-down
Advantages
Thermosiphon:

Circulation is continuous
as long as the sun is
shining

Larger pipe size

Sloped

Do not rely on pumps
and controllers
Direct circulation systems:

Can be used with city
water
Indirect water heating systems:

For areas with extensive
freezing
Air water-heating systems:

Used for preheating
domestic water

Reliable

Longer life than forced
circulation systems

Does not need protection
from freezing or boiling

Do not require electricity

Non-corrosive

Cost-effective
Heat pump system:

Higher coefficient of
performance
Pool heating systems:

Requires no separate
storage tank

Inherent freeze
protection
Disadvantages
Thermosiphon:

Tall units (not
aesthetically pleasing)

Cannot handle hard or
acidic water

Suitable for mild
climates with mild freeze
conditions only
Integrated collector storage:

High thermal losses

Slow temperature change

More expensive and less
efficient than passive
systems

More difficult to retrofit
in houses

Cannot handle hard or
acidic water
Direct circulation systems:

Use in areas with
infrequent freezing

Power failure = system
freeze
Indirect water heating systems:

Expensive to construct
and maintain
Air water-heating systems

Takes a lot of space

Difficult to detect leaks

Parasitic power
consumption is higher
Heat pump systems:

Requires special
equipment and expertise
to maintain

Limited heat transfer
Pool heating systems:

Large area required

Solar absorption is
dependent on the color,
depth, and pool usage

Heat loss is affected by
evaporation, radiation,
air temperature, wind
speed, relative humidity,
vapor pressure, simmer
turbulence, conduction to
the ground, and rainfall
In all solar water heating systems, thermal storage is one of its main components. “Because for
approximately half the year any location is in darkness, heat storage is necessary if the solar system must
operate continuously…Usually the design and selection of the thermal storage equipment is one of the
most neglected elements of solar energy systems. It should be realized, however, that the energy storage
system has an enormous influence on overall system cost, performance, and reliability” (Kalogirou, 2009,
p. 275). The thermal storage unit can be located indoors or outdoors. If it is located outdoors it should be
elevated above the ground or on the roof; however, it is recommended that the unit be located indoors for
decreased heat loss. Furthermore, thermal storage units come in two types: air (which use rocks as the
most common media for thermal storage) and liquid (which use domestic water or a separate fluid for
thermal storage and use).
With any system “the amount of how water produced by a solar water heater depends on the type
and size of the system, the amount of sunshine available at the site, and the seasonal hot water demand
pattern” (Kalogirou, 2009, p. 252). Sometimes the amount of hot water required necessitates the
connection of multiple modules to create an array of solar water heaters; nevertheless, regardless of the
system size, there are several considerations that designers need to take into consideration. These
considerations include:

Orientation and tilt of the solar collection unit

Shading

Thermal expansion

Galvanic corrosion

Array sizing

Heat exchangers

Pipe heat losses

Over-temperature protection

Freeze protection

Sensor locations

Instrument use
(Image retrieved March 18, 2012 from http://www.solarfeeds.com/wpcontent/uploads/solar-hot-water.jpg)
The International Standards Organization (ISO) has developed a set of standards for testing domestic
solar water heater performance (ISO 9459-1 through ISO 9459-5). The potential for solar water heating
has been barely realized, as of 2005 “about 140 million m2 of solar thermal collector area were in
operation around the world, which is only 2.3% of the potential” (Kalogirou, 2009, p. 251). This shows
how extensively solar energy could possibly effect our everyday lives in a sustainable manner.
Chapter 5 Questions
5.7 Repeat Example 5.3 for September 15, considering that the weather conditions are the same.
5.9 A liquid-based solar heating system uses a heat exchanger to separate the collector loop from the
storage loop. The collector overall heat loss coefficient is 6.3 W/m2-°C, the heat removal factor is 0.91,
and the collector area is 25 m2. The heat capacity rate of the collector loop is 3150 W/°C and, for the
storage loop, is 4950 W/°C. Estimate the thermal performance penalty that occurs because of the use of
the heat exchanger if its effectiveness is 0.65 and 0.95.
5.10 A liquid-based solar heating system uses a heat exchanger to separate the collector loop from the
storage loop. The flow rate of the water is 0.65 kg/s and that of the antifreeze is 0.85 kg/s. The heat
capacity of the antifreeze solution is 3150 J/kg-°C and the UA value of the heat exchanger
is 5500 W/°C. The collector has an area of 60 m2 and an F R U L = 3.25 W/m2-°C. Estimate the factor F’
R/F R.
5.14 A solar collector system has a total area of 10 m2, F R = 0.82, and U L =7.8 W/m2-°C. The collector
is connected to a water storage tank of 500 L, which is initially at 40°C. The storage tank loss coefficientarea product is 1.75 W/°C and the tank is located in a room at 22°C. Assuming a load flow of 20 kg/h and
a make-up water of 18°C, calculate the performance of this system for the period shown in the following
table and check the energy balance of the tank.
Hour
7-8
8-9
9-10
10-11
11-12
12-13
13-14
14-15
15-16
16-17
17-18
S (MJ/m2)
0
0.35
0.65
2.51
3.22
3.56
3.12
2.61
1.53
0.66
0
Ta (C)
12.1
13.2
14.1
13.2
14.6
15.7
13.9
12.1
11.2
10.1
9.2
Chapter 6: Solar Space Heating and Cooling
“The two principle categories of building solar heating and cooling systems are passive and
active. The term passive system is applied to buildings that include, as integral parts of the building;
elements that admit, absorb, store, and release solar energy and thus reduce the needs for auxiliary energy
for comfort heating. Active systems are the ones that employ solar collectors, storage tanks, pumps, heat
exchangers, and controls to heat and cool the building” (Kalogirou, 2009, p. 315) For both systems the
calculation of the building’s thermal load (“the rate at which energy must be added o removed from a
space to maintain the temperature and humidity at the design values” (Kalogirou, 2009, p. 316)) is
important. “Many methods can be used to estimate the thermal load of buildings. The most well-known
are the heat balance, weighing factors, thermal network, and radiant time series” (Kalogirou, 2009, p.
315). These methods attempt to estimate the effects of various factors which directly affect building heat
gains. Some of these factors include:

Wall and roof heat transfer

Partitions, ceilings, and floors

Glazing (or glass)

People

Lighting

Appliances and

Ventilation and infiltration air
(Building interior image retrieved March 17, 2012 from
http://www.socketsite.com/Cathedral%20Building%20Interior.jpg)
Passive Space Heating Systems
“Passive solar heating systems require little, if any, non-renewable energy to function. Every
building is passive in the sense that the sun tends to warm it by day and it losses heat at night. Passive
systems incorporate solar energy collection, storage, and distribution into the architectural design of the
building and make (Image retrieved March 18, 2012 from http://www.energyrant.com/ minimal or no use of
mechanical equipment, such
wp-content/uploads/2009/02/passive-solar-design.jpg)
as fans, to deliver the collected energy. Passive
solar heating, cooling, and lighting design must consider the building envelope and its orientation and
design, the use of sun spaces, and natural ventilation.
As part of the design process, a preliminary analysis must be undertaken to investigate the
possibilities for saving energy through solar energy and the selection of the appropriate passive technique.
The 1st step to consider for each case investigated should include an analysis of the climatic date of the
site and definition of the comfort requirements of the occupants and the way to meet them. The passive
system can then be selected by examining both direct and indirect gains” (Kalogirou, 2009, p. 328).It can
consist of:

Thermal storage walls

Rectangular building shape with the long axis oriented in the direction of east-west

Shading (deciduous trees, awnings, blinds, etc.)

Insulation

Sunspaces

Overhangs

Ventilation and

Strategically placed windows and internal walls (Image retrieved March 18, 2012 from
http://www.ecohomedesigns.org/eco_home_design_pics/passive_solar_02.jpg)
Active Space Heating Systems
Active space heating systems consist of the use of solar panels and other solar systems which are
externally controlled and added onto the building infrastructure. They include solar systems for the
heating of the building, electricity/energy needs, and domestic hot water production.
(Image retrieved March 18, 2012 from http://farm4.staticflickr.com/3027/2367427103_f760cfe197.jpg)
Solar Cooling
The use of solar energy for household cooling needs (comfort cooling and refrigeration) has been
much less investigated and applied than the use of solar energy for household heating needs. Nonetheless,
“alternative cooling technologies that can be applied to residential and commercial buildings, under a
wide range of weather conditions, are being developed. These include night cooling with ventilation,
evaporative cooling, desiccant cooling and slab cooling. The design of buildings employing low-energy
cooling technology, however, presents difficulties and requires advanced modeling and control techniques
to ensure efficient operation…In effective solar energy control, summer heat gains must be reduced,
while winter solar heat gains must be maximized. This can be achieved by proper orientation and shape of
the building, the use of shade devices, and the selection of proper construction materials” (Kalogirou,
2009, p. 361) It thus becomes evident that the combining of solar heating and cooling systems can create
highly efficient solar energy units. As Kalogirou writes, “the greatest disadvantage of a solar heating
system is that a large number of collectors need to be shaded or disconnected during summertime to
reduce overheating. A way to avoid this problem and increase viability of the solar system is to employ a
combination of space heating and cooling and domestic how water production systems. This is
economically viable when the same collectors are used for bath space heating and cooling” (2009, p. 381)
References
Kalogirou, S. (2009). Solar Energy: Processes and Systems (1st ed.). London, UK: Academic Press:
Elsevier