Transcript Slide 1

Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Part I
D. Yogi Goswami, Frank Kreith, Jan F. Kreider
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Outline
Introduction
 Calculations of Heating and Hot Water Load in Buildings.
 Calculation of Heat Loss
 Internal Heat Sources in Buildings
 The Degree-day Method
 Service Hot-Water Load Calculation
 Solar Water Heating Systems
 Natural Circulation Systems
 Forced-Circulation Systems
 Industrial Process Heat Systems
 Liquid-Based Solar-Heating Systems for Buildings
 Physical Configurations of Active Solar Heating Systems3
 Solar Collector Orientation
 Fluid Flow Rates
 Thermal Storage
 Other Mechanical Components
 Controls in Liquid Systems
 Load Devices in Liquid Solar-Heating Systems
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Introduction
Historically, methods used for collecting and transferring solar heat were passive
methods, that is, without active means such as pumps, fans and heat exchangers.
Passive solar heating methods utilize natural means such as radiation, natural
convection, thermosyphon flow and thermal properties of materials for collection
and transfer of heat.
Active solar heating methods, on the other hand, use pumps and fans to enhance
the rate of fluid flow and heat transfer.
This chapter describes in detail the function and design of active systems for
heating buildings and service water.
Energy for heating buildings and hot water consumes about one-fourth of the
annual energy production in the United States.
In many areas of the United States and the world, solar heating can compete
economically with other types of fuel for heating, without even considering the
environmental benefits.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heating and Hot Water Load in Buildings
5.1 Calculations of Heating and Hot Water Load in Buildings
Energy requirements for space heating or service water heating can be calculated from
basic conservation of energy principles.
For example, the heat required to maintain the interior of a building at a specific
temperature is the total of all heat transmission losses from the structure and heat
required to warm and humidify the air exchange with the environment by infiltration and
ventilation.
ASHRAE has developed extensive heat load calculation procedures embodied in the
ASHRAE Handbook of Fundamentals.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heating and Hot Water Load in Buildings
Heat loss calculations for buildings
Figure on the right shows the combinations of
temperature and humidity that are required for
human comfort. The shaded area is the
standard U.S. comfort level for sedentary
persons.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heat Loss
 The components of heat loss calculations of a building are given below.
Heating load calculations for buildings
Heating load component
Walls, Roof, Ceilings, Glass
Equations
Descriptions/References
𝑞 = 𝑈 ∙ 𝐴(𝑇𝑖 − 𝑇𝑜 )
𝑈𝑓 has special units of 𝑊/𝑚2 . Values of 𝑈𝑓
for various ground water temperatures are
given in Appendix 5.
Basement floors and walls below ground level
𝑞𝑓 = 𝑈𝑓 ∙ 𝐴𝑓
Concrete floors on ground
𝑞𝑓𝑒 = 𝐹𝑒 𝑃𝑒 (𝑇𝑖 − 𝑇𝑜 )
𝑞𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒 = 𝑄𝜌𝑎 𝐶𝑝𝑎 (𝑇𝑖 − 𝑇𝑜 )
or = 1200 ∗ 𝑄(𝑇𝑖 − 𝑇𝑜 ) Watts
Infiltration and ventilation air
𝑘𝑔
are inside and outside air
𝑇𝑖 , 𝑇𝑜
temperature, respectively. 𝑈 values of
composite section are calculated from the
thermal properties of components given in
Appendix 5.
𝑞𝑙𝑎𝑡𝑒𝑛𝑡 = 𝑄𝜌𝑎 ℎ𝑓𝑔 ∆𝑊
or = 2808 ∗ 𝑄∆𝑊 Watts
𝑃𝑒 is the perimeter of the slab. 𝐹𝑒 values are
given in Appendix 5.
𝑄 is volume of air flow in
𝜌𝑎
and
𝐶𝑝𝑎 are density and specific heat of air.
ℎ𝑓𝑔 is the latent heat of water at room
temperature.
∆𝑊 is humidity ratio difference between
inside and outside air.
𝐽
Assuming 𝜌𝑎 = 1.2 3 ; ℎ𝑓𝑔 = 2340 𝐽/𝑘𝑔, 𝐶𝑝𝑎 = 1000 𝑘𝑔 ℃
𝑚
D. Y. Goswami, F. Kreith, J. F. Kreider
𝑚3
.
𝑠𝑒𝑐
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heat Loss
 Transmission heat losses through attics, unheated basements, and the like are
buffered by the thermal resistance of the unheated space. For example, the
temperature of an unheated attic lies between that of the heated space and that of
the environment.
 As a result, the ceiling of a room below an attic is exposed to a smaller temperature
difference and consequent lower heat loss than the same ceiling without the attic
would be.
 The effective conductance of thermal buffer spaces can easily be calculated by
forming an energy balance on such spaces.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heat Loss
Example. Calculate the heat load on a house for which the wall area is 200 m2, the floor
area is 600 m2, the roof area is 690 m2, and the window area totals 100 m2. Inside wall
height is 3 m. The construction of the wall and the roof is shown below.
Cross-sections of the wall and the roof for the Example
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heat Loss
Selective
Surfaces
Example. The thermal
resistance of the wall can be found by the electrical resistance
analogy as:
Rwa = Routside air + Rwood siding + Rsheathing + Rcomb + RWall board + Rinside air
Combined thermal resistance for the studs and insulation (Rcomb) is found as:
1
𝑅𝑐𝑜𝑚𝑏
𝐴𝑠𝑡𝑢𝑑 𝐴𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛
1
=
+
𝑅𝑠𝑡𝑢𝑑 𝑅𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝐴𝑠𝑡𝑢𝑑 + 𝐴𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛
Assuming that the studs occupy 15% of the wall area,
1
0.15 0.85
=
+
𝑅𝑐𝑜𝑚𝑏
0.77 1.94
or
𝑅𝑐𝑜𝑚𝑏 = 1.58
D. Y. Goswami, F. Kreith, J. F. Kreider
𝑚2℃
𝑊
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heat Loss
Reflecting
Surfaces
Therefore the wall resistance,
R , can be found as:
wa
Specular reflectance values
𝑈𝑤𝑎 =
1
1
=
= 0.46 𝑊/𝑚2 ℃
𝑅𝑤𝑎 2.179
The heat loss through the windows depends on whether they are single-glazed or
double-glazed. In this example, single-glazed windows are installed, and a U factor equal
to 4.7 W/m2 oC is used.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heat Loss
Transparent
The roof is constructed of 12.7 mm gypsum wall board, 51 mm foam insulation board,
38 mm still air, 12.7 mm plywood, and asphalt shingles (wooden beams and roofing
Materials
paper are neglected for the simplified calculations here). Therefore,
𝑈𝑟𝑓
1
=
= 0.32 𝑊/𝑚2 ℃
0.030 + 0.077 + 0.11 + 0.17 + 2.53 + 0.079 + 0.1
Outside air
Shingles
Plywood
Air Gap
Foam
Wallboard
Inside air
If the respective areas and U factors are known, the rate of heat loss per hour for
the walls, windows, and roof can be calculated.
𝑊
Walls:
qwa=(200 m2) x 0.46 𝑚2 ℃ = 92 𝑊/℃
𝑊
Windows:
qwi=(100 m2) x 4.7 𝑚2 ℃ = 470 𝑊/℃
Walls:
qrf=(690 m2) x 0.32 𝑚2 ℃ = 220 𝑊/℃
+____________
Total qtr =782 𝑊/℃
𝑊
If double-glazed windows were used, the heat loss would be reduced to 552 𝑊/℃.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heat Loss
The infiltration and ventilation rate Q for this building is assumed to be 0.5 ACH
(Air Changes per hour).
𝑞𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒 = 𝑄𝜌𝑎 𝐶𝑝𝑎 (𝑇𝑖 − 𝑇𝑜 )
Sensible heat load
𝑞𝑙𝑎𝑡𝑒𝑛𝑡 = 𝑄𝜌𝑎 ℎ𝑓𝑔 ∆𝑊
Latent heat load
In residential buildings, humidification
of the infiltration air is rarely done, so
latent heat load is neglected.
𝑄 = 0.5 × (600 𝑚2 × 3 𝑚) = 900 𝑚3 /ℎ𝑟 = 0.25 𝑚3 /𝑠
volume
𝑞𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒
𝑚3
𝑘𝑔
𝐽
= 0.25
× 1.2 3 × 1000
℃ = 300 𝑊/℃.
𝑠
𝑚
𝑘𝑔
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Calculations of Heat Loss
Therefore,
𝑞𝑡𝑜𝑡 = 𝑞𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒 + 𝑞𝑡𝑟 = 782 + 300 = 1082 𝑊/℃
 This calculation is simplified for purposes of illustration. Heat losses through the slab
surface and edges have been neglected, for example.
 More refined methods of calculating energy requirements on buildings do not use
the steady-state assumption used above [23].
 The thermal inertia of buildings may be expressly used as a load-leveling device. If
so, the steady-state assumption is not met and the energy capacitance of the
structure must be considered for accurate results.
 Many adobe structures in the U.S. Southwest are built intentionally to use daytime
sun absorbed by 1-ft-thick walls for nighttime heating for example.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
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Chapter 5: Solar Heating Systems
Internal Heat Sources in Buildings
Heat supplied to a building to offset energy losses is derived from both the heating
system and internal heat sources.
Some common internal sensible heat gains that tend to offset the heating
requirements of buildingsa
aFor
more data see [2].
factor is the amount of a window not in a shadow expressed as a decimal between 1.0 and 0.0.
bShading
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
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Chapter 5: Solar Heating Systems
Internal Heat Sources in Buildings
 Commercial buildings such as hospitals, computer facilities, or supermarkets will
have large internal gains specific to their function.
 Internal heat gains tend to offset heat losses from a building but will add to the
cooling load of an air-conditioning system.
 The magnitude of the reduction in heating system operation will be described in the
next section.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
The Degree-day Method
 The preceding analysis of heat loss from buildings expresses the loss on a per unit
temperature difference basis (except for unexposed floor slabs).
 In order to calculate the peak load and total annual load for a building, appropriate
design temperatures must be defined for each.
 The outdoor design temperature is usually defined statistically, such that the actual
outdoor temperature will exceed the design temperature 97.5 % or 99 % of the time
over a long period.
 The design temperature difference (∆𝑇) is then
∆𝑇 = 𝑇𝑖𝑛𝑡𝑒𝑟𝑖𝑜𝑟 𝑏𝑢𝑖𝑙𝑑𝑖𝑛𝑔
−
𝑇𝑜𝑢𝑡𝑑𝑜𝑜𝑟
 The design ∆𝑇 is used for rating non-solar heating systems, but is not useful for
selection of solar systems, since solar systems rarely provide 100 % of the energy
demand of a building at peak conditions.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
The Degree-day Method
 A more useful index of heating
energy demand is the total annual
energy requirement for a building.
This quantity is somewhat more
difficult to calculate than the peak
load.
Building load profile versus ambient temperature
showing no-load temperature Tnl and desired interior
temperature Ti.
 It requires a knowledge of day-to-day
variations in ambient temperature
during the heating season and the
corresponding building heat load for
each day.
 Building heat loads vary with
ambient temperatures as shown at
the figure.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
The Degree-day Method
 The environmental temperature Tnl, above which no heat need be supplied to the
building, is a few degrees below the required interior temperature Ti because of
internal heat-generation effects.
 The no-load temperature at which internal source generation qi just balances
transmission and infiltration losses can be determined from the energy balance
𝑞𝑖 = 𝑈𝐴 𝑇𝑖 − 𝑇𝑛𝑙
where 𝑈𝐴 is the overall loss coefficient for the building (W/oC).
Then,
𝑇𝑛𝑙 = 𝑇𝑖 −
𝑞𝑖
𝑈𝐴
The total annual heat load on the building, QT, can be expressed as
𝑄𝑇 =
365 𝑑𝑎𝑦𝑠
+ indicates that only positive values are considered.
D. Y. Goswami, F. Kreith, J. F. Kreider
𝑈𝐴(𝑇𝑛𝑙 − 𝑇𝑎) + 𝑑𝑡
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
The Degree-day Method
In practice, it is difficult to evaluate this integral, therefore, three simplifying
assumptions are made:
1. 𝑈𝐴 is independent of time.
2. 𝑇𝑛𝑙 is independent of time.
3. The integral can be expressed by the sum.
Thus,
365
𝑈𝐴
𝑇𝑛𝑙 − 𝑇𝑎
𝑛
+
𝑛=1
Where n is the day number, and the daily average temperature 𝑇𝑎 can be approximated
by
(𝑇𝑎, 𝑚𝑎𝑥 − 𝑇𝑎, 𝑚𝑖𝑛)/2
in which Ta,max and Ta,min are the daily maximum and minimum temperatures,
respectively.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
The Degree-day Method
 The quantity 𝑇𝑛𝑙 − 𝑇𝑎
+ is
called the degree-day unit.
 For example, if the average ambient temperature for a days is 5oC and the no-load
temperature is 20oC, 15 degree C-days are said to exist for that day.
 However, if the ambient temperature is 20oC or higher, 0 degree-days exist,
indicating 0 demand for heating that day.
 Degree-day totals for monthly ( 𝑚𝑜𝑛𝑡ℎ 𝑇𝑛𝑙 − 𝑇𝑎 𝑛+ ) and annual periods can be
+
used in 𝑈𝐴 365
𝑛=1 𝑇𝑛𝑙 − 𝑇𝑎 𝑛 to calculate the monthly and annual heating energy
requirements.
 In the past, a single value of temperature has been used throughout the United
States as a universal degree-day base, 65.0°F or 19°C, however, since many
homeowners and commercial building operators have lowered their thermostat
settings in response to increased heating fuel costs, thereby lowering Tnl, this
practice is now outdated.
 Therefore, a more generalized database of degree-days to several bases (values of
Tnl) has been created by the U.S. National Weather Service (NWS).
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
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Chapter 5: Solar Heating Systems
The Degree-day Method
Example. A building located in Denver, CO,
has a heat-loss coefficient 𝑈𝐴 of 1000
kJ/hr°C and internal heat sources of 4440
kJ/hr. If the interior temperature is 20°C
(68°F), what are the monthly and annual
heating energy requirements?
Monthly and annual energy demands
A gas furnace with 65 % efficiency is used to
heat the building.
Solution. In order to determine the monthly
degree-day totals, the no-load temperature
(degree-day basis) must be evaluated,
4440
𝑇𝑛𝑙 = 20 −
= 15.6℃ (60 ℉)
1000
The monthly degree C-days for Denver are
taken from the U.S. National Weather Service
and given in the table.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
The Degree-day Method
The energy demand is calculated as,
Monthly and annual energy demands
ℎ𝑟
𝑈𝐴 × 24
× Degree C−days
𝑑𝑎𝑦
The annual energy demand of 62.9 GJ is
delivered by a 65% efficient device.
Therefore,
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐴𝑛𝑛𝑢𝑎𝑙 𝑃𝑢𝑟𝑐ℎ𝑎𝑠𝑒𝑑 𝐸𝑛𝑒𝑟𝑔𝑦,
62.9
𝐺𝐽
0.65
= 96.8 𝐺𝐽
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Service Hot-Water Load Calculation
 Service hot-water loads can be calculated precisely with the knowledge of only a few
variables.
 The data required for calculation of hot-water demand are,
 Water source temperature
(Ts)
 Water delivery temperature (Td)
 Volumetric demand rate
(Q)
 The energy requirement for service water heating qhw is given by,
𝑞ℎ𝑤 𝑡 = 𝜌𝑤𝑄 𝑡 𝑐𝑝𝑤 𝑇𝑑 − 𝑇𝑠 𝑡 ,
Where 𝜌𝑤 is the water density and 𝑐𝑝𝑤 is its specific heat.
 The demand rate, Q(t), varies in general with time of day and time of year; likewise,
the source temperature varies seasonally.
 Source temperature data are not compiled in a single reference; local water
authorities are the source of such temperature data.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Service Hot-Water Load Calculation
Example. Calculate the monthly
energy required to heat water for a
family of four in Nashville, TN.
Monthly source temperatures for
Nashville are shown in Table 5.5, and
the water delivery temperature is 60°C
(140 °F).
Approximate service hot-water demand rates
Solution. For a family of four, the
demand rate Q may be found using a
demand recommended from the
table.
𝑙𝑖𝑡𝑒𝑟𝑠
𝑚3
𝑄 = 4 × 76
= 0.30
𝑑𝑎𝑦
𝑑𝑎𝑦
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Service Hot-Water Load Calculation
The density of water can be taken as 1000
kg/m3 and the specific heat as 4.18 kJ/kg °C.
Monthly demands are given by,
Approximate service hot-water demand rates
𝑑𝑎𝑦𝑠
𝜌𝑤𝑐𝑝𝑤 𝑇𝑑 − 𝑇𝑠 𝑡
𝑚𝑜𝑛𝑡ℎ
𝑑𝑎𝑦𝑠
= 0.30 ×
103 × 4.18 60 − 𝑇𝑠 𝑡
𝑚𝑜𝑛𝑡ℎ
𝑞𝑚 = 𝑄 ×
The monthly energy demands calculated
from the equation above with these data
are tabulated in the next slide.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Service Hot-Water Load Calculation
Water heating energy demands for the example
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Solar Water Heating Systems
Solar Water Heating Systems
 Solar water-heating systems represent the most common application of solar energy
at the present time.
 There are basically two types of water-heating systems:
 natural circulation or passive solar system (thermosyphon)
 Forced circulation or active solar system
 Natural circulation solar water heaters are simple in design and low cost.
 Their application is usually limited to nonfreezing climates, although they may also
be designed with heat exchangers for mild freezing climates.
 Forced circulation water heaters are used in freezing climates and for commercial
and industrial process heat.
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Chapter 5: Solar Heating Systems
Natural Circulation Systems
 The natural tendency of a less dense fluid to rise above a denser fluid can be used in
a simple solar water heater to cause fluid motion through a collector.
 The density difference is created within the solar collector where heat is added to
the liquid.
Schematic diagram of thermosyphon loop used in a natural circulation
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
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Chapter 5: Solar Heating Systems
Natural Circulation Systems
 As water gets heated in the collector, it rises to the tank, and the cooler water from
the tank moves to the bottom of the collector, setting up a natural circulation loop. It
is also called a thermosyphon loop.
 For the thermo syphon to work, the storage tank must be located higher than the
collector.
 The flow pressure drop in the fluid loop (∆𝑃) must equal the bouyant force "pressure
difference" (∆𝑃𝐵𝑂𝑈𝑌𝐴𝑁𝑇 ) caused by the differing densities in the hot and cold legs of
the fluid loop.
∆𝑃𝐹𝐿𝑂𝑊 = ∆𝑃𝐵𝑂𝑈𝑌𝐴𝑁𝑇
𝐿
= ρ𝑠𝑡𝑜𝑟𝑔𝐻 − [ 0 𝜌 𝑥 𝑔𝑑𝑥 + 𝜌𝑜𝑢𝑡𝑔(𝐻 − 𝐿)]
where H is the height of the legs and L is the height of the collector, 𝜌 𝑥 is the local
collector fluid density, ρ𝑠𝑡𝑜𝑟 is the tank fluid density, and ρ𝑜𝑢𝑡 is the collector outlet fluid
density, the latter two densities assumed uniform.
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Principles of Solar Engineering
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Chapter 5: Solar Heating Systems
Natural Circulation Systems
The flow pressure term, ∆𝑃𝐹𝐿𝑂𝑊, is related to the flow loop system head loss, which is
in turn directly connected to friction and fitting losses and the loop flow rate:
∆𝑃𝐹𝐿𝑂𝑊 =
𝜌𝑑(ℎ𝐿)
𝐿𝑂𝑂𝑃
where ℎ𝐿 = 𝐾𝑉2, with K being the sum of the component loss velocity factors and V the
flow velocity.
Since the driving force in a thermosyphon system is only a small density difference and
not a pump, larger-than-normal plumbing fixtures must be used to reduce pipe friction
losses.
Since the hot-water system loads vary little during a year, the angle of tilt is that equal
to the latitude, that is, 𝛽 = 𝐿.
The temperature difference between the collector inlet water and the collector outlet
water is usually 8-11°C during the middle of a sunny day.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Natural Circulation Systems
Passive solar water heaters
compact model using combined collector
and storage
section view of the compact model
tank and collector assembly
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Natural Circulation Systems
After sunset, a thermosyphon system can reverse its flow direction and lose heat to the
environment during the night.
To avoid reverse flow, the top header of the absorber should be at least 30 cm below
the cold leg fitting on the storage tank, as shown, otherwise a check valve would be
needed.
To provide heat during long cloudy periods, an electrical immersion heater can be used
as a backup for the solar system.
The immersion heater is located near the top of the tank to enhance stratification and
so that the heated fluid is at the
required delivery temperature.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Natural Circulation Systems
Several features inherent in the thermo syphon design limit its utility.
If it is to be operated in a freezing climate, a nonfreezing fluid must be used, which in
tum requires a heat exchanger between collector and potable water storage.
Heat exchangers of either the shell-and-tube type or the immersion-coil type require
higher flow rates for efficient operation than a thermo syphon can provide.
Therefore, the thermo syphon is usually limited to nonfreezing climates.
For mild freeze climates, a heat exchanger coil welded to the outer surface of the tank
and filled with an antifreeze may work well.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Natural Circulation Systems
Example 5.4. Determine the "pressure difference" available for a thermosyphon system
with 1 meter high collector and 2 meter high legs. The water temperature input to the
collector is 25°C and the collector output temperature is 35°C. If the overall system loss
velocity factor (K) is 15.6, estimate the system flow velocity.
Solution. ∆𝑃𝐹𝐿𝑂𝑊 is solved with the water densities being found from the steam tables
(see Appendix 3 and Tables 3.8 and 3.9).
𝐿
∆𝑃𝐹𝐿𝑂𝑊 = ∆𝑃𝐵𝑂𝑈𝑌𝐴𝑁𝑇 = ρ𝑠𝑡𝑜𝑟𝑔𝐻 − [
0
𝜌 𝑥 𝑔𝑑𝑥 + 𝜌𝑜𝑢𝑡𝑔(𝐻 − 𝐿)]
ρ𝑠𝑡𝑜𝑟(25℃) = 997.009 𝑘𝑔/𝑚3
ρ𝑠𝑡𝑜𝑟(35℃) = 994.036 𝑘𝑔/𝑚3
ρ𝑐𝑜𝑙𝑙𝑒𝑐 𝑎𝑣𝑒 30℃ = 996.016𝑘𝑔/𝑚3
(𝑛𝑜𝑡𝑒: 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑢𝑠𝑒𝑑 𝑖𝑛 "𝑖𝑛𝑡𝑒𝑔𝑟𝑎𝑙)
𝐻 = 2 𝑎𝑛𝑑 𝐿 = 1 𝑚:
∆𝑃𝐵𝑂𝑈𝑌𝐴𝑁𝑇 = 997.009 × 9.81 × 2 − [(996.016) × 9.81 × 1 + (994.036) × 9.81 × 1)]
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Natural Circulation Systems
∆𝑃𝐵𝑂𝑈𝑌𝐴𝑁𝑇 = 38.9 𝑁/𝑚2(𝑃𝑎)
The system flow velocity is estimated from the system K given, the pressure difference
calculated above, taking the average density of the water around the loop (at 30°C), and
substituting into,
∆𝑃𝐵𝑂𝑈𝑌𝐴𝑁𝑇 = ρ𝑐𝑜𝑙𝑙𝑒𝑐 𝑎𝑣𝑒 ℎ𝐿
𝑉2
=
𝑙𝑜𝑜𝑝
= ρ𝑐𝑜𝑙𝑙𝑒𝑐 𝑎𝑣𝑒 𝐾𝑉2
38.9
996.016 15.6
𝑉 = 0.005 𝑚/𝑠
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Forced-Circulation Systems
Open loop system
In an open loop system the solar loop is at atmospheric pressure, therefore, the
collectors are empty when they are not providing useful heat.
A disadvantage of this system is the high pumping power required to pump the water to
the collectors every time the collectors become hot.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Forced-Circulation Systems
Closed loop system
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Forced-Circulation Systems
Closed loop drain-back system
This disadvantage can be
overcome in a closed loop
drain-back system which is
not pressurized.
In this system, when the
pump shuts off, the water in
the collectors drains back
into a small holding tank
while the air in the holding
tank goes up to fill the
collectors.
The holding tank can be
located where freezing does
not occur, but still at a high
level to reduce pumping
power.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Forced-Circulation Systems
 In all three configurations, a differential controller measures the temperature
differential between the solar collector and the storage, and turns the circulation
pump on when the differential is more than a set limit (usually 5°C) and turns it off
when the differential goes below a set limit (usually 2°C).
 Alternatively, a photovoltaic (PV) panel and a DC pump may be used.
 The PV panel will turn on the pump only when solar radiation is above a minimum
level.
 Therefore, the differential controller and the temperature sensors may be eliminated.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Industrial Process Heat Systems
For temperatures of up to about 100°C, required for many industrial process heat
applications, forced circulation water-heating systems described above can be used.
The systems, however, will require a large collector area, storage and pumps, etc.
For higher temperatures, evacuated tube collectors or concentrating collectors must be
used.
Industrial process heat systems are described in more detail in Chapter 8.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Solar Water Heating Systems
Liquid-Based Solar Heating Systems for Buildings
 The earliest active solar space-heating systems were constructed from enlarged
water-heating components.
 Solar space-heating systems can be classified as active or passive depending on the
method utilized for heat transfer
 A system that uses pumps and/or blowers for fluid flow in order to transfer heat
is called an active system
 On the other hand, a system that utilizes natural phenomena for heat transfer is
called a passive system
 Passive solar heating systems are described in Chapter 7.
 In this section, configurations, design methods, and control strategies for active
solar-heating systems are described.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Solar Collector Orientation
Typical solar-thermal system for space heating and hot-water heating showing fluid transport loops and
pumps.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Solar Collector Orientation
 The collector fluid loop contains
fluid manifolds, the collectors,
the collector pump and heat
exchanger, an expansion tank,
and
other
subsidiary
components.
Typical solar-thermal system for space heating and hotwater heating showing fluid transport loops and pumps.
 The storage loop contains the
storage tank and pump as well
as the tube side of the collector
heat exchanger.
 To capitalize on whatever
stratification may exist in the
storage tank, fluid entering the
collector heat exchanger is
generally removed from the
bottom of storage.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Solar Collector Orientation
 The best solar collector
orientation is such that the
average solar incidence angle is
smallest during the heating
season.
Typical solar-thermal system for space heating and hotwater heating showing fluid transport loops and pumps.
 For tracking collectors this
objective
is
automatically
realized.
 For fixed collectors in the
northern hemisphere the best
orientation is due south (due
north
in
the
southern
hemisphere), tilted up from the
horizon at an angle of about
15° greater than the local
latitude.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Fluid Flow Rates
 Although high flows maximize energy
collection, practical and economic
constraints put an upper limit on
useful flow rates.
Effect of fluid flow rate on collector performance
as measured by the heat-removal factor FR; F‘ is
the plate efficiency factor.
 Very high flows require large pumps
and excessive power consumption
and lead to fluid conduit erosion.
 In practice, liquid flows in the range
of 50-75 kg/hr mc2 (10-15 Ib/hr ftc2)
D of water equivalent are the best
compromise among collector heattransfer coefficient, fluid pressure
drop and energy delivery.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Thermal Storage
 Experience has shown that liquid storage amounts of 50-75 kg H2O/mc2 (10-15 lb/ft2)
are the best compromise between storage tank cost and useful energy delivery.
Effect of liquid storage capacity on liquid-based
solar-heating system energy delivery
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Controls in Liquid Systems
 Control strategies and
hardware
used
in
current solar system
designs are quite simple
and are similar in
several respects to
those
used
in
conventional systems.
 The single fundamental
difference lies in the
requirement
for
differential temperature
measurement instead of
simple
temperature
sensing.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Controls in Liquid Systems
 The collector-storage difference is
sensed by two thermistors or
thermocouples, the difference
being determined by a solid-state
comparator, which is a part of the
control device.
 Room temperature is sensed by a
conventional
dual
contact
thermostat.
 The control system operates as
follows;
 If the first room thermostat contact
closes, the mode selector valve and
distribution pump are activated in
an attempt to deliver the thermal
demand
from
solar-thermal
storage.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Controls in Liquid Systems
If room temperature continues to
drop, indicating inadequate solar
availability, the mode selector diverts
flow through the backup system
instead of the solar system, and the
backup is activated until the load is
satisfied.
The
collector-storage
control
subsystem operates independently of
the heating subsystem described
above.
If collector temperature exceeds the
temperature in the bottom of the
storage tank by 5-10oC, the collector
pump and heat-exchanger pump are
activated and continue to run until
the
collector
and
storage
temperature are within about 1-2oC.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Load Devices in Liquid Solar-Heating Systems
 A heating load device transfers heat from the solar storage to the air in the space.
 Therefore, a liquid-to-air heat exchanger is sized based on the energy demand of a
building.
 Several generic types of load devices are in common use;
1. Forced-air systems-tube-and-fin coil located in the main distribution duct of a
building or zone of a building.
2. Baseboard convection systems-tube-and-fin coils located near the floor on
external walls. These operate by natural convection from the convectors to the
room air.
3. Heated floors or ceilings-water coils. These transfer heat to large thermal
masses that in tum radiate or convect into the space. This heating method is
usually called radiant heating.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering
Principles of Solar Engineering
Chapter 5: Solar Heating Systems
Load Devices in Liquid Solar-Heating Systems
Forced-air heating system load device location upstream of non-solar heat exchanger or furnace.
D. Y. Goswami, F. Kreith, J. F. Kreider
Principles of Solar Engineering