Problem Set 5

Question 1

What are (a) ballast and (b) rack-mounted PV systems? What kind of building would they be
appropriate for?

Answer 1

(a) Ballast-mounted PV systems are held down by gravity (the weight of the
ballast) .

(b) Rack-mounted systems are attached to the roof (usually bolted).

Question 2

List three ways that watertightness can be achieved in sloped roof BIPV systems?

Answer 2

i. Using overlapping tiles or shingles.
ii. Using a watertight layer below the PV.
iii. Using rubber seals between PV modules.
iv. Attaching the PV to a watertight standing-seam metal roof (preferably with a good air-gap for
ventilation)

Question 3

What kinds of BIPV façades allow optimal tilt or orientation (when the building orientation is not optimal)?

Answer 3

Accordion style BIPV facades will allow optimal tilt, while sawtooth BIPV facades will allow for optimal orientation of the array.

Question 4

Under what circumstances can a PV façade be ventilated?

Answer 4

A BIPV façade that is part of a double wall system, such as rainscreen cladding or an atrium system can be ventilated by providing inlets at the bottom of the facade and outlets at the top.

Question 5

Discuss the use of double glazed or insulated BIPV façade panels and BIPV façade panels with a lowemissivity
back surface. What are the implications of using such products in terms of PV performance
and effect on the interior temperature of the building?

Answer 5

Double glazed BIPV façade panels will insulate the back surface of the module, so that conduction heat transfer will be reduced. Low-E back surfaces on PV modules will reduce heat transfer by radiation from the back surface of the module. Both will reduce heat transfer into the building interior, but as they will not allow as much heat to be dissipated from the module’s back surface, will result in higher cell temperatures. Better PV and building performance can often be achieved if the PV façade is ventilated and the heat used to reduce heating loads (in winter) and dumped outside the building to reduce cooling loads (in summer).

Question 6

Your client wants to cover their roof with BIPV tiles. There is a ceiling below the roof. If the roof cavity
is ventilated, how will this affect the temperature of the PV and the temperature in the rooms below?
How could you minimise the impact of the temperature of the roof cavity on the rooms below?

Answer 6

If a roof cavity is ventilated, heat transfer from the modules will be increased, reducing the operating temperature of the modules. The temperature of the roof cavity will be reduced in most circumstances (unless Ta is higher than the temperature in the roof). This may cause the heat loss from the rooms below, which may not be desirable. Insulating the ceiling using bulk and/or reflective foil insulation can prevent heat loss from the building interior to the cooler roof.

Question 7

Compare the likely importance of the following issues for roof and façade BIPV installations, and
discuss the effect on system design decisions of each:

• (a) temperature effects

Answer 7

PV modules have reduced efficiency at high temperatures. Temperatures in roofs are generally higher than in facades, but roof cavities can often be ventilated. Rainscreen cladding can also be ventilated, but conventional facades are difficult to ventilate without
compromising the comfort of the interior. The effect of high temperatures on the building interior can be reduced by insulating the back surface of modules or using low-E surfaces, but the modules will operate at higher temperatures

Question 8

Compare the likely importance of the following issues for roof and façade BIPV installations, and discuss the effect on system design decisions of each:

• (b) solar access (tilt, orientation and shading)

Answer 8

Close to optimal tilt and orientation can occur on tilted roofs. Ballast and rack-mounted systems generally used for flat roofs can be designed for optimal tilt and orientation. Tilted roofs may also be at reasonably good tilt angles. Vertical facades will not have optimal tilt,
although in locations further from the equator, these will be closer to optimal. Sawtooth or accordion facades can be used to achieve optimal orientation or tilt in facades. Shading from objects at the horizon is more likely to affect façade arrays or arrays located lower down on roofs. If shading cannot be avoided, modules that experience similar shading at similar time should be connected in series. Losses due to low PV efficiency at low levels of irradiation are lower with amorphous technology, which can be considered when there are
only non-optimal locations available for PV.

Question 9

Compare the likely importance of the following issues for roof and façade BIPV installations, and discuss the effect on system design decisions of each:

(c) partial shading

Answer 9

Partial shading will often be caused by objects close to the horizon and will hence be more significant with façade arrays or arrays located lower down on roofs. Partial shading may also be caused by services penetrating the roof, such as chimneys or vents. Partial
shading on arrays should be studied and if the partial shading is unavoidable, only modules that experience similar partial shading should be connected in series. Thin-film modules can be considered in cases of variable partial shading, since they are less affected than crystalline modules by partial shading effects, because the cells run the entire length of the
module.

Question 10

Compare the likely importance of the following issues for roof and façade BIPV installations, and discuss the effect on system design decisions of each:

(d) appearance and visibility

Answer 10

Façade mounted arrays, semi-transparent glass (e.g. in atriums, windows or skylights) and shading devices will be the most visible. Tilted roof arrays may also be visible, but flat roof arrays will often not be visible from the ground. High visibility may be desirable if one of the
aims is to project a green or high-tech image. In some circumstances, a discreet PV system is desirable. Thin film technologies tend to have a more uniform appearance, while multicrystalline modules usually have blue AR-coatings on the cells. Semi-transparency can
be achieved in glass-glass crystalline PV modules and through pin-holes in thin-film modules. Each will result in a distinctive appearance.

Question 11

Compare the likely importance of the following issues for roof and façade BIPV installations, and discuss the effect on system design decisions of each:

(e) placement of cabling

Answer 11

Cabling from PV modules should be protected from weather and, ideally, not visible. Mounting systems can be designed to hide cables and protect them from the weather. Cables often need to penetrate the building envelope, in which case attention should be paid to making the penetration watertight. Shorter lengths of DC cabling will result in smaller voltage drops and power losses along the cables. If the inverter is placed closer to
the array, the length of the cables can be reduced. When modules are placed in series, higher voltages will result. When modules are in parallel, higher current will result. Power (I2 R losses) will be higher when currents are higher. Low current (series connection) is therefore preferable, but increases susceptibility to partial shading. Thicker cables have a lower resistance and should be used when high currents cannot be avoided.

Question 12

Compare the likely importance of the following issues for roof and façade BIPV installations, and discuss the effect on system design decisions of each:

(f) access for maintenance

Answer 12

PV arrays may need to be replaced in the event of a failure or breakage. BOS components such as inverters also need to be accessible for monitoring, maintenance or replacement. The design of the mounting system and the placement of the BOS components should allow for access for maintenance. Systems with many inverters allow detailed monitoring of sub-arrays, and provide redundancy but may be difficult to maintain.

Question 13

How can transparency be achieved in crystalline and amorphous PV modules?

Answer 13

Transparency in amorphous modules is achieves via pin-holes in the PV film. In crystalline modules, the spaces between the cells can be transparent if the encapsulant and backing materials are transparent. The transparency can be adjusted by changing the number or size of pin-holes in an amorphous module, or adjusting the spacing between crystalline cells. Light that is transmitted through the module will not contribute to generating current in the module.

Question 14

Why do solar cells operate hotter than the ambient air temperature?

Answer 14

The solar cell only converts some of the incident radiation to electricity (10% for a 10% efficient solar
cell). Of the rest of the solar radiation, about 5% will be lost by reflection and shading from the front metal contacts, while the remainder that is absorbed by the panel and not converted to electricity is converted to heat.

Question 15

What does NOCT mean? Why is it significant?

Answer 15

NOCT is the Nominal Operating Cell Temperature, which is the temperature the solar cell will reach if allowed to reach equilibrium at ambient temperature of 20˚C, wind speed 1 m/s and 0.8 kW/m2 irradiance.

NOCT gives us a measure of how hot the module will get in the field, since different modules will generate and dissipate heat at different rates. A module that operates
very hot in the field will have a much lower maximum power point than the rated MPP, and hence produce less power. 

FORMULA

Question 16

Describe the appearance of single crystal, multicrystalline and amorphous pv modules.

Answer 16

Single crystal solar cells are circular or square (with cut corners), often around 10 × 10cm and waferthin. They are grey, and are covered in a metal grid which collects current. Modules are made up of 36 or more solar cells arrayed flat in a regular pattern, connected together by metal tape, and sandwiched in glass and transparent plastic materials. Multicrystalline cells are square and have visible crystals in the silicon, so have a sparkly effect. They are generally blue, due to an antireflection coating. Amorphous solar cells are made in large, flat sheets and a transparent conductor is used instead of a
metal grid. They hence form smooth, flat, grey-black glass covered panels. The transparent oxide layer sometimes gives an interesting metallic sheen.

Question 17

List the drawbacks and benefits of amorphous modules compared to crystalline modules in an
architectural context.

Answer 17

 less efficient
 efficiency less affected by temperature
 able to be deposited on many different substrates
 cheap production processes
 light degradation from initial exposure
 partial shading less extreme
 smooth, uniform appearance
 uncertainty in product lifetime (less proven)
 less visible, so less projection of ‘green’ image

Question 18

What are some of the benefits to the user and the utility of grid connected pv systems?

Answer 18

 Grid ensures availability of power to user and in effect acts as storage (in a similar way to a bank
storing your money, except hopefully without fees).
 No battery needed – excess energy produced contributes to grid – avoid power losses in conversion
of energy to chemical potential & back.
 ‘Self-sufficiency’ for user – insulated from electricity price fluctuations – may even be a net producer
of electricity.
 Peak shaving can reduce need to use expensive gas powered generators at peak times (demandside
management).
 Distributed generators can alleviate need to invest in larger capacity generation & transmission
equipment & avoids losses
 Reliability of distribution grid can be improved by redundancy of many small generators distributed
throughout the network.

Question 19

A building is affected by shading from near objects on the roof. Since the temperatures in this climate
are extremely hot, it is inadvisable to put module integrated inverters on the roof. Suggest an
alternative inverter configuration that would reduce shading losses.

Answer 19

Shading losses can be reduced by using a string configuration, with an efficient central inverter located
elsewhere (in a cooler place). This could, however, lead to large DC wiring runs, so another alternative would be a multi-string inverter configuration, where each of the strings has it’s own DC-DC converter stage which can include MPPT functionality for the string. A central inverter may then be located in a suitable place and if the wiring run is lone, the voltage of the DC bus leading to the inverter can be higher (stepped up by the DC-DC converters) to avoid large voltage drops and power loss in the DC bus.

Question 20

PV efficiency is reduced from the rated efficiency when operating at temperatures above STC. What is
affected more Voc or Isc? How does this affect the design of string configuration in an array? How
important is this effect compared to the effect of partial shading when designing a string configuration?
Discuss the difference in temperature effects between amorphous and crystalline silicon cell
technologies.

Answer 20

In crystalline modules, Voc is affected more than Isc. Isc has a slightly positive temperature coefficient, but it is not significant. Voc is reduced by on average 2.3 mV per ˚C increase in operating cell temperature for crystalline modules. Since the MPP voltage will be different for modules operating at different temperatures, they should
ideally not be connected in parallel, otherwise they will not be operating at MPP. If modules were connected in series strings, mismatch due to temperature effects is negligible, since identical modules at different temperatures will have similar currents. Partial shading in strings is much more important, since the current produced by modules shaded differently will be different. Even if modules at different temperatures are connected in parallel, the effect is much less important than module mismatch in strings caused by partial shading. When modules are connected in series,
the current is limited to that of the module with the lowest current, whereas when modules with different voltages are connected in parallel, the open circuit voltage across them will be between the open circuit voltage of the two & hence is not as significant. 

The negative temperature coefficients for power and voltage are much smaller for amorphous modules than for crystalline modules. Some amorphous modules even have a positive power temperature coefficient. Temperature effects are hence much less significant for amorphous modules.

Question 21

Why are grid-connected inverters often undersized?

Answer 21

Due to high operating temperatures and soiling, modules will rarely operate at their rated output. Inverters are less efficient at low power. In order to cause the inverter to operate in it’s more efficient (higher power) range, inverters are commonly undersized compared to the Wp rating of the array.

Question 22

What three derating factors are used to derate module output when sizing PV systems?

Answer 22

ftemp (temperature derating factor), fdirt (soiling derating factor) and fmanf (derating factor for manufacturers
tolerance on output).

Question 23

Why are module temperatures often higher in BIPV systems than in conventionally mounted PV
systems?

Answer 23

BIPV mounting systems often result in low ventilation rates. BIPV roof mounted systems may back onto the roof cavity, which is much hotter than the ambient temperature, or the building interior, which may also be hotter than the ambient temperature. Additionally, some module mounting structures include insulation or low-E surfaces, which prevent heat dissipation from the back surface of the module.

Question 24

Discuss the difference in system monitoring between a string inverter configuration and a central
inverter configuration

Answer 24

System monitoring is usually done by measuring the output at the inverter(s). Central inverters do not allow easy monitoring of individual modules or strings, since data is only obtained from the whole array. A problem can be very difficult to find (each module / connection needs to be checked individually to find a fault) and may go unnoticed in a large array.

Since there is more than one inverter in a string inverter configuration, each inverter must be monitored, to ensure that the system is operating correctly. Communication from the inverter will allow the performance of each of the strings to be easily monitored. Finding a problem in a single module or connection requires checking each one in the string, rather than the whole array. A larger number of inverters adds complexity to the system monitoring, but the monitoring is more effective in finding a bad
inverter (or module)

Question 25

Under what circumstances would you choose (a) a central inverter configuration (b) a module-integrated
inverter for a BIPV system? Why?

Answer 25

(a) Central inverters will have a higher efficiency than module-integrated inverters, but do not allow maximum power point tracking of individual modules. If modules in the array experience very similar levels of irradiation (same orientation & tilt) and there are no partial shading problems, central inverters are a good solution.
(b) Module integrated inverters are suitable in instances where there is a high degree of variability or uncertainly about irradiation levels and partial shading, since each module will have it’s own maximum power point tracker and hence mismatch losses can be avoided. If there are areas of the array that are oriented or tilted differently, but no partial shading, string inverters may be a better solution, since their efficiency is higher, they cost less ($), and the inverter need not be exposed to extreme weather (temperatures and humidity).

Question 26

If the inverter fails in a central inverter system, your system will not feed any power to the grid or the
building. What can be done to provide redundancy so that the system does not go down completely,
without resorting to smaller inverters?

Answer 26

The use of master-slave configurations ensures high system availability. If one inverter goes down, the
system will switch to the other inverter.

Question 27

List four reasons that modules may have a different current output in a PV system. Should modules
with different current outputs be connected in series or parallel? Why?

Answer 27

Modules may have different current output due to partial shading, the use of modules with different characteristics, different orientation or tilt, or different amounts of soiling. Modules with different current output should not be connected in series if possible, since the current through the worst module will limit the current through the rest of the series string.

Question 28

If two rows of identical modules are installed in a roof, such that the row of modules higher up
experience higher temperatures due to stratification, should you connect the modules that are at a
higher temperature in series or parallel with those at lower temperature?

Answer 28

Identical modules that are at a higher temperature will have a reduced maximum power point voltage. If they are connected in parallel, they will not operate at their maximum power point, but at a voltage between the VMPPs of the separate modules. Since the current output at MPP of the separate modules will be very similar, if they are connected in series, the output of the combination will not be reduced significantly. They would be better connected in series, although the significance of temperature with regards to mismatch is much less than partial shading, which should be the major consideration.

Question 29

What are the major considerations when choosing a location for an inverter in a BIPV system?

Answer 29

Inverters sometimes make noise, so large inverters should not be installed to close to frequently occupied parts of a building. Inverters should not be exposed to extremely high or low temperatures (see manufacturer’s specifications) or high levels of humidity, so should be protected from weather and high levels of solar radiation and well ventilated. Installing inverters close to the array will reduce the length of DC cabling, which will reduce losses in the cabling and the need to use thick cables.

Question 30

What specifications does your inverter need to fit in terms of output from the PV?

Answer 30

The inverter must be able to handle the maximum power expected from the array. The maximum Voc expected from the array (at the lowest temperature expected) must also be below the absolute maximum voltage rating of the inverter. The maximum and minimum expected MPP voltages from the array should be within the MPP voltage window of the inverter.

It is common to assume that the maximum output expected from the array will be less than 80% of the rated output, due to derating factors (particularly temperature derating), but it is critical to check whether
the array would go outside the inverter’s power or voltage limits and to be aware of the inverter’s behaviour when power or voltage limits are exceeded before choosing an inverter. Note that inverters sometimes limit the output from the array to the upper limit of the voltage or power window by moving the operating point of the array, but they sometimes cut the array production entirely

Question 31

Why is it important to monitor the performance of PV systems?

Answer 31

 Know when the system has a fault.
 Judge whether the design goals have been met.
 Determine the performance, reliability and durability of the plant and its components.
 Compare PV installations of different sizes and different applications, operating in different climatic
conditions.
 Provide data for a general assessment of the potential of PV technology and the improvement of
system design and operation.

Question 32

What is the Array Yield (Ya)?

Answer 32

Array yield is the ratio of the daily energy from the array to the rated power of the array

Question 33

Final Yield

Answer 33

The Yf is the ratio of the daily energy from the PV system (delivered to the load) to the rated power
of the array. It can be used to compare PV systems independently from their size.

Question 34

Reference Yield

Answer 34

The reference yield Yr represents the solar energy ‘theoretically’ available per kilowatt peak of installed PV per day.

Question 35

Array capture losses

Answer 35

All losses before the inverter input

Question 36

System losses

Answer 36

Losses between the power conditioning (inverter & MPPT or regulator/battery) input and output.

Question 37

Performance Ratio

Answer 37

Enables the comparison of grid connected PV systems independently from their location, tilt angle, orientation and nominal power (system size).

Question 38

Why do outages due to inverter failures appear in monitoring statistics as array capture losses, rather
than system losses?

Answer 38

Lc = Yr - YA
Since the output from the array will be not be measured at the inverter input (the inverter has failed), the losses appear as array capture losses

Question 39

What are the factors that contribute to poor array performance (efficiency)? Which loss factors appear
statistically as array capture losses that are actually not losses in the array?

Answer 39

Array performance losses:
 temperature effects
 low irradiance
 wiring, string diode losses
 partial shading, contamination, snow covering, non-homogenous irradiance
 soiling
 spectral losses, reflection losses
 non-optimal orientation & tilt (not evaluated by PR)
 Manufacturers tolerance on output
 Bypass diodes
Factors that appear statistically as array losses in grid-connected systems, but are not due to the array:
 maximum power point tracking errors
 reduction of array power caused by inverter failures

Question 40

Which losses are measured by System Losses?

Answer 40

Inverter conversion losses in grid-connected systems and from battery storage losses in stand-alone systems. (Oversized inverters can increase system losses).

Question 41

What are the major BOS losses in BIPV systems?

Answer 41

 Inverter inefficiency
 MPPT losses
 Inverter failures
 DC wiring losses

Question 42

What are the major causes of low energy yields revealed by extensive performance monitoring of BIPV
systems?

Answer 42

 Failures of systems or components
 Shading of arrays
 Bad orientation of arrays
 High module temperatures
 Incorrect inverter sizing

Question 43

What is the most unreliable and the most reliable component in a grid-connected system? Is this
changing over time?

Answer 43

The most unreliable component is the inverter, while the most reliable component is the modules. Although inverter reliability is increasing, inverters remain the biggest source of system failures, while the modules are still the most reliable part of the system.

Question 44

Indicate the range of high, average and low PRs obtained for monitored PV systems. Is the PR of
systems improving or getting worse? Why?

Answer 44

Average PRs are around 70-75. Systems with PRs below 70 have high losses (often due to system component failures, shading or temperature effects.

Question 45

Attention to what aspects of BIPV system design will lead to more optimal performance?

Answer 45

 Reliable components and quality installations
 Avoiding shading, partial shading and mismatch by using optimal layout
 Optimal orientation & tilt
 Good ventilation
 Frequent system inspections or good monitoring
 Correct sizing of inverter