Modelling of thermal energy storage to increase the
penetration of renewable energy in electricity production
Manuel Ngola Cusseiala
[email protected]
Instituto Superior Técnico, Lisboa, Portugal
May 2013
Abstract
Solar thermal systems produce hot water by harnessing energy from the sun. Portugal
has excellent conditions for the utilization of this energy source, so the use of solar collectors for
water heating has grown significantly in recent years as it uses can reduce significantly energy
costs and environmental impacts.
In a context where systems have to be optimized to maximize the economic and
technical efficiency, solar thermal systems can aid, albeit indirectly, in the integration of
renewables for electricity generation. In general, in the winter months, solar thermal systems
require the use of another energy source to support water heating, namely gas or electricity. In
the case the support is electric, solar thermal systems can then be used as an indirect storage
system, using the backup when there is excess of renewable electricity generation and thus
prevent its use during periods of the day when the use of backup power is less interesting. The
use of solar thermal in this way can be advantageous for example in residential buildings that
aim to be net zero energy buildings. Thus, this thesis presents a hourly model of a conventional
solar thermal system used in the residential sector and compares the modeling results with
Solterm, the reference software for Portugal.
Key-Words: Renewables Energies, Solar Thermal Systems, Solterm, Hourly Model
1. Solar Thermal Systems
possibility of placing or not the storage tank
at a higher level than the solar collector.
It is defined as thermal solar system a
device that heats water from the sun. This
A solar thermal system, consists
system has two main components: the solar
essentially
collector for solar energy capture and
of
the
solar
collector,
accumulator (storage tank), support system
storage tank for hot water. These two
and the hydraulic circuit.
components can be interconnected with or
without circulation pump, depending on the
1
2. Modelling
proportional to the negative temperature
gradient
The energy received by the solar
thermal system is described by the energy
exchanged by radiation between the sun
The constant k is the thermal conductivity of
and the panel.
the material. Between two substances,
which have higher conductivity able to
The energy emitted by radiation, that is,
transfer a greater amount of heat for a
the emissive power of the surface is given
given temperature difference.
by
The
exchange
of
heat
by
convection is given by Newton's law, and
the heat flow is
Where
σ
is
the
Boltzmann
constant
)
(
2.1.
The flow of energy received by a surface
Model Applied
n simplified form, the e
per unit area is given by the following
ations of energy
balance can be obtained considering a
expression:
solar collector absorber plane traversed by
a mass flo
Where IN is the intensity of radiation normal
m . The following figure shows
schematically the energy balance made the
to the surface. Your unit in the SI system is
absorber.
W / m ^ 2.m.sr (power per unit area per
length and per unit solid angle).
And
Conservation laws and radiation,
provide well-defined relationships between
the radiative properties of the bodies. In the
Figure 1-Absorsor modeling system
case only of radiant energy, the total energy
received has to be reflected, transmitted or
The amount of heat captured is given by
absorbed into the body to maintain its
temperature. Thus, the relation between the
Having regard to the consideration of a flat
reflectivity (ρ), the transmittance (τ) and the
surface, or with a form factor equal to zero,
absorptivity (α) of a given body, for a given
the
wavelength is
radiation) is directly proportional to incident
absorbed
energy
(due
to
direct
radiation. If the surface is not flat or convex,
The heat flux (heat exchanged per unit
that is, the form factor different from zero,
area) is given by Fourier's law, which states
there may be an increase in power
that the flow of heat through a material is
consumption (if the radiation reflected by
2
2.2.
the surface back to the point of focus on the
Generic Model
surface) - concave or wavy.
Not every incident energy (QI) is absorbed
by the panel (QA). Some of the energy is
reflected (QR), the others are exchanged by
conduction-convection
(QCC)
with
the
surrounding air with a temperature T_a and
emitted radiation (QE).
Figure 2-Global model system
From figure 1, the following equations
The amount of energy entering the system
result:
is given by
Or
and that goes for
The term QI-QR corresponds to the radiant
here ΔQ is the variation of the energy
energy absorbed by the surface of the
system, QSupport is the energysupport, QC
panel absorber, QCC + QE represents the
the energy captured by the collector, QD
energy loss by the total area of the panel
energy dissipated by heat, the energy
itself and by radiation and convection QA is
required QN, QP heat losses, Q Pe heat
absorbed energy, ie the energy transmitted
losses in the pipes outside QPi heat losses
to the fluid heat. The exchanges of energy
in the pipes and indoor QPd thermal losses
by conduction, convection and radiation
in the tank and QAg energy associated with
energy emitted by the sun are given by:
water compensation. The energy support is
in addition to the energy of heating water
(QAq) performed by an auxiliary equipment
The radiant
(electrical
energy absorbed by the
resistance,
eg),
the
energy
pumping (QB) in the case of forced
absorber panel surface is given by
circulation.
Finally, the energy absorbed by the panel is
he energy required (QN) is calculated
given by
starting from the expression
When the panel temperature equal to
Nd which represents the number of days per
ambient temperature, ie when there are no
month, VAQS the volume of water consumed
radiative and convective losses to the
per
environment (QI-QR = QA), the yield of the
person,
is
the
temperature
difference and Nd is the number of days per
panel is equal to the optical performance
year:
and is termed initial yield.
3
solar radiation but also has the deposit
accumulated energy to meet the power
needs for some time (period 6 in the figure).
When this energy is exhausted, goes into
Considering a course similar to the energy
operation again support system (period 1 of
captured radiation received during the day
the figure). Briefly, the example of Figure 4
can thus be graphically represent the
shows that the storage tank is slightly
exchanges of energy and heating needs. In
undersized and which allow a greater
the event of energy requirements and
capacity May 1st accumulation of energy
losses are constant during the day (Figure
and a reduction in power system to provide
4), beginning the day the heat required is
support.
provided by the support system (period 1 in
The previous figure can be subdivided into
the figure). When the panel begin receiving
two figures representing the solar system
solar radiation, the first instants (period 2 in
and serving the same system with the same
the figure) is used for heating the system
which
was
due
to
cold
needs and Time periods but with different
ambient
use. As can be seen, the accumulated heat,
temperatures overnight, until it reaches the
dissipated provided by the deposit and the
preset temperature.
support system are different in each case.
Figure 3-Heat exchange: constant energy needs in daily
functioning
For a time, the needs are met by the heat
Figure 4-Heat exchanges: energy needs in the
period of solar radiation
energy captured by the panel and the
excess is stored in the storage tank (period
In this figure, the initial instants (one period
3 in the figure). When the temperature
of the figure) when the radiation starts to
preset temperature limit is reached, any
focus the panel serve to warm up the
excess heat is dissipated (period 4 in the
system
figure). In this case the heat captured by
until
it
reaches
the
preset
temperature. Since there is no energy
the panel is not sufficient to meet their
needs, the heat captured is stored (period 2
energy needs, but the heat accumulated in
in the figure). Subsequently the energy
the tank is enough to compensate for this
captured by the panel serve to meet the
difference (period 5 in the figure). At the
needs and the excess heat is accumulated
end of the day, the panel fails to capture
in the tank (period 3 in the figure). After the
4
preset temperature is reached, all heat is
the end of the day, when the panel no
dissipated (period 4 in the figure). In the
longer receive solar radiation, energy needs
period 5, the heat captured by the panel is
are supplied by the energy stored in the
no longer sufficient to meet the energy
tank (period 5 in the figure). To exhaust the
requirements, but the energy accumulated
accumulated energy, all heat requirements
in the tank compensates the deficit of
are met by the support system (period 6 in
energy. After that there needs heat, solar
the figure). Unlike the others, this storage
panel system, to fail to capture solar
tank is undersized.
radiation, heat builds up in the tank (period
2.3.
6 in the figure). In this case, contrary to the
Model Time
This analysis is based on two important
above, there is need for support system,
equations which follow:
but it is to emphasize the importance of
energy captured by the panel have been
dissipated: this system was oversized.
Where QAt represents the accumulated
In the second case (Figure 6), the moments
energy at time t, QAbt the energy absorbed
in which radiation begins to focus on the
in the moment, QAb(t-1) the energy absorbed
panel, the first moments serve to make the
at the last instant, QApt energy support the
heating system until it reaches the preset
instant t, QN is the necessary energy
temperature (period 1 of the figure). In
consumption at time t. If QApt <0, has
period 2 the heat captured is stored in the
accumulated more energy than necessary,
tank as soon as there is no energy needs.
however, there is no need for support but
When the preset temperature is reached, all
energy dissipation if necessary, ie QApt = 0.
the excess heat is dissipated (period 3 in
the figure). Subsequently, there needs to
2.4.
Implementation of the Model
heat initially met these are partly captured
The
by the energy and the other panel by the
spreadsheet, Figure 6 represents the model
energy accumulated by the container (4
discussed in the month, and figure 7 shows
period of the figure).
a part of the time model, i.e., energy
model
was
implemented
in
a
analysis of all hours of the year, a total of
8760 hours .
Figura 5-Heat exchanges: energy needs at night
5
2.5.
Calculation SOLTERM
program
Solterm is a program for performance
analysis of solar thermal and photovoltaic,
specially tuned to weather conditions and
techniques
of
Portugal.
Performance
analysis of a solar system is made via
energy simulation under quasi-stationary:
ie, are simulated energy balance in the
system at short intervals (5 minutes), during
which
considers
steady
state
of
the
environment and the system. In these
simulations are used information about:
Configuration / system design
Figure 6-Monthly model
• Strategies for control and operation
• Horizontal solar radiation and temperature
on hourly basis
• Obstr ctions, shading, alebdo nearby,
turbidity of the atmosphere
• Technical characteristics of components
(collectors, storage, etc.).
• Cons mption (or "load") system in an
hourly average monthly
Figure 7-Daily model
6
The program has a friendly interface and
easy to understand, figure 8 represents the
section in which the properties of the
chosen system, dedicated to the RCCTE in
Figure 9 shows the section in which the
choice
is
made
of
weather
data.
Figure 11 - Analysis of results
Where:
Rad.Horiz. - Accumulated energy (monthly
or
annual)
global
solar
radiation
on
horizontal surface, per unit area (kWh / m²).
The global radiation is the direct sum of the
components (from the direction of the sun)
Figure 8-Workplace program Solterm and data
selection
and diffuse (coming from the celestial
hemisphere and reflected from the ground
and close to the ground surface) radiation.
Rad.Inclin. - Accumulated energy (monthly
or annual) of solar radiation to the face of
solar collectors per unit area (kWh / m²), so
an inclined plane. Note that this value has
the effect of modifying included angle of
incidence.
Figure 9-Pick-climatic conditions in the program
Solterm
Wasted - accumulated energy (monthly or
In Figure 10 we define the consumption
annual) that collects the solar system but it
profiles, as well as the temperatures of
has to dissipate (kWh). Energy waste
water intrusion. Finally, in Figure 11 we
collected is almost always because they
obtain the results of the simulation of the
exceed the temperature limits for water
energy analysis of the problem
storage in situations where consumption is
small or zero. This value should not be
confused with the thermal losses in tanks,
pipes, etc..
Provided - accumulated energy (monthly or
annual) that the system provides for
consumption (kWh). It is useful final energy,
ie effectively delivered. This value is called
Figure 10 - Choice of consumption profile
7
eSolar Energy in Building Regulations, vd.
supports energy, as can be seen in the
Decree-Law no. 80/2006, of 4 April.
following figure:
Load - cumulative (monthly or annual)
energy required for consumption (kWh).
Support - accumulated energy (monthly or
annual) delivered for consumption by the
support system or help to supplement the
energy provided by the solar system (kWh).
Figure 12-Profile Consumption
It is useful final energy: the energy value
corresponding final will be higher, and even
In
more the value of primary energy.
consumption of a family of adults and
profile
youth,
in
1,
profile
especially
2
was
morning
considered
consumption split between morning and
Solar fraction - this is the percentage of
night (families with children) and lastly,
useful energy supplied for consumption
profile 3, use throughout the day, ie family
from solar radiation (reason "Delivered" /
with people at home to noon.
"Load" in annual values). It is therefore the
Case 1
contribution of the solar system itself for the
consumer requested. The solar fraction is
When the DHW consumption is anticipated
the
performance
in the design system, solar collectors
evaluation for solar thermal systems. In
provide enough energy to supply about
general seeks to achieve a solar fraction
80% of the needs and work with an
(annual) between 40% and 90%. Below this
adequate
range, the system is generally undersized,
conducted for average daily consumption of
above this range is often that is oversized.
160 liters of DHW, the energy required is
However, these are only guide values for
3040 kWh / year and the contribution of
typical situations. In many cases, such as
solar collectors are 2738 kWh / year. The
nightly loads or loads strongly seasonal
energy will be supporting this annual event
(concentrated in the summer or winter), this
of 302 kWh / year, having a solar fraction of
statement is not appropriate. In any case it
0.90.
primary
measure
of
income.
In
this
example,
is always insufficient use annual solar
fraction as the only criterion for sizing.
3. ANALYSIS OF RESULTS AND
DISCUSSIONS
3.1.
In
this
consumption profiles
thesis,
we
consider
three
Figure 13 - Consumption profile: 160l
consumption profiles that require different
8
Case 2
significant increase going to have a value of
When consumption is lower than expected,
694 kWh / year. As a conclusion it can be
the contribution of solar collectors will also
said that the solar collector is typically sized
be lower, but will have a greater impact on
to meet about 50-70% of hot water needs
overall accounting. In case 2 as shown in
during the period of one year. The intakes
Figure 14, a daily intake of 120 liter DHW,
have great influence in direct energy
the energy required would be 2280 kWh /
benefits of solar panels.
year, while the collectors contribute with the
same amount of 2738 kWh / year, which is
more 100% of demand, thus signifying that
the system is oversized. In this situation,
the operating temperatures would be higher
and the performance would be lower
collectors and solar fraction well above
Figure 15-Consumption profile: 200l
unity. Normal. The solution to this problem
lies in the reduction of energy absorbed,
Table 1-comparison of results
reducing the area of the collectors.
Global
annual
[kWh]
Figure 14 - Consumption profile: 120l
Monthly
Daily
Profile 1
Profile 2 Profile 3
Qprovided
2738
2733
2733
1920
Qneeded
3040
3066
3066
3066
Qsupport
302
539
545
717
Sf
0,9
0,89
0,89
0,62
Sf is the solar fraction.
As we can see in the table shown above,
there are no significant differences in
Case 3
energy
Finally, when consumption is higher than
between
the
model
provided
monthly schedule, in profiles 1 and 2, but 3
expected, the solar collectors provide more
for the profile already exists significant
energy, although their total contribution is
differences. This is due to the fact that the
smaller. In Figure 15, for a consumption of
model time, energy is accumulated every
200 liters per day, the energy required
hour, and when it is used, ie when it
would be 3800 kWh / year, and the
exceeds the required temperature, needs
constant contribution of solar collectors,
more energy dissipation, which is why it has
2738 kWh / year, which will represent about
been higher energy support in the model
70% of satisfaction of needs through the
than in the time monthly. Overall, in terms
solar system . Operating temperatures are
of solar fraction divergence exists only in
lower and the yield collectors tend to
profile 3, due to energy consumption in
increase. The energy will also support a
times of high radiation.
9
The profile 3 has lower energy supplied
4. CONCLUSION AND FUTURE
WORK
due to consume energy at the same time
being be heated.
In view of the arguments presented, the
Sun is a clean energy source and free.
Table 2-cost analysis per day
Solar energy during use is environmentally
Cost [€/day]
P1
P2
P3
friendly unlike energy from fuels that are not
Yearly
only
limited,
their
use
pollutes
the
Wi
Sp
Su
Au
[€]
ST
1,10
0,35
0
0,18
75,09
BT
1,23
0,39
0
0,22
83,60
TT
1,24
0,40
0
0,22
84,27
ST
1,10
0,58
0
0,18
75,98
BT
1,23
0,65
0
0,20
84,60
increasingly solar energy an economical
TT
1,24
0,67
0
0,23
87,12
and environmentally friendly. Installation on
ST
1,10
0,21
0
0,18
99,93
a small scale does not require large
BT
1,23
0,23
0
0,21
111,26
investments in transmission lines due to
TT
1,21
0,24
0
0,23
112,85
being excellent in difficult locations.
environment. The power generation of solar
energy
require
minimal
maintenance
compared to other energy sources. Solar
panels are evolving in terms of power while
their cost decreases. This is becoming
The
climatic
conditions
cause
large
variations in the amount of solar energy
P1, P2 and P2 are the diferente profiles
production, since overnight output is zero,
used. Wi, Sp, Su and Au represente the
which requires the use of means of energy
season of the year, Winter, spring, summer
storage produced during the day. The ways
and autumn respectively, ST is the Silmple
of storing solar energy are inefficient when
tariff, BT is bi-hourly tariff na TT is tri-hourly
compared to energy from fossil fuels,
tariff.
hydropower and biomass.
In terms of energy costs, we can
observe that the simple tariff will be
In Portugal, specifically in the mainland, the
cheaper than other rates, the rate being
use of solar energy is essential due to high
less economic tri-hourly tariff as seen in the
solar radiation presents,
table below represented. The prices are
facilitates the reduction of energy demand
relative to the total annual higher and lower
and thus energy loss would occur in the
in profile 3 of profile 1, this is because, as
transmission.
and
its use
calculated above, the support power is
greater in profile 3 and low in the profile 1,
It is suggested that future work, study of
while the second profile similar to the profile
increasing the efficiency of collecting solar
1 values due to similarity between daily
energy, using panels with the swivel
energy consumption.
position of the sun throughout the day.
10
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