Howto Photovoltaics

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en ew a b l eEn er g y

“How to” Manual

Walter HulshorstEcon International

January 2008

ResidentialPhotovoltaic Systems



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www.leonardo-energy.org

Introduction

The sun is an abundant and readily available source of energy. A photovoltaic

system, known more familiarly as PV or solar panels, captures the sun’s energy

and converts it into usable electricity.

In fact, PV systems are already an important part of all our lives. Simple PV

systems, for example, power many smaller consumer items, such as calculators

and wristwatches. More sophisticated systems power communications satellites

and water pumps, and power the appliances and lights in many homes and

workplaces. PV is a renewable energy source that can be installed easily, even

in existing homes, which makes PV systems very attractive to urban dwellers.

Figure 1: Residential PV installations [1]

How to use this manual

This manual provides basic information to those who are considering installing a

photovoltaic (PV) system in their home, at their office or in other types ofbuildings. With a PV system, you have a quiet, environmentally friendly,

electricity-producing “power plant”. Choosing a PV system also makes a strong

statement in support of ecologically responsible, sustainable energy. PV

technology will also likely play a significant role in meeting our future energy

needs.



“How to” Manual to Residential Photovoltaic (PV) Systems

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Whatever of these reasons applies to you, this manual will help you decide

whether PV is a viable option for you. This guide will:

• give a basic explanation of how PV works;

• describe some of the main components of a PV system;

• offer ideas on the design and placement of a PV system that is right for

you;

• outline how to determine if PV makes sense for you.

One important point to bear in mind is that, for a PV system to be most effective,

a house or building must already be energy efficient. The less energy a home or

building uses, the fewer PV panels will be needed, and thus the smaller the initial

investment.

How PV systems work: An overview

Photovoltaic (PV) systems work by converting sunlight directly into electricity, by

using what are known as ‘solar cells’. A solar cell is made of semi-conducting

material in two layers: P and N (see figure 2). When radiation from the sun hits

the photovoltaic cell in the form of sunlight, the boundary between P and N acts

as a diode: electrons can move from N to P, but not the other way around.

Photons with sufficient energy hitting the cell cause electrons ( ) to move fromthe P layer into the N layer. An excess of electrons builds up in the N layer while

the P layer builds up a deficit. The difference in the amount of electrons is the

voltage difference, which can be used as a power source. As long as light

continues to hit the panel, the voltage difference is maintained; even on cloudy

days, due to diffuse radiation of the light.

Figure 2: Schematic overview of the operation of a photovoltaic cell [2]



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The amount of electric power that a photovoltaic produces depends principally on

two factors:

• the amount of incident sunlight;• the efficiency of the photovoltaic in converting this light into electricity.

This output is specified as the total nominal DC solar panel output, under

standard test conditions, in accordance with IEC 61215; for example, illumination

of 1 kW/m2, cell temperature of 25o C. Output efficiency of crystalline PV arrays

decreases by 0,5 per cent per degree Celsius over the standard test temperature

of 25° C. Proper ventilation is required at the back of modules. In determining the

placement modules, exposure to cooling breezes is an important consideration.Specialists in the field of PV do not express the installed power of a system in

watts (W) but in watt-peak (Wp).

A residential PV system enables a homeowner to generate some or all of their

daily electrical energy demand on their own roof, exchanging daytime excess

power for future energy needs, usually for night time use. The house remains

connected to the public electricity grid at all times, thus any power required

above what the PV systems can produce is drawn from the grid. PV systems can

also include battery backup or uninterruptible power supply (UPS) to operateselected circuits in the residence for hours or days during a grid outage.

The emphasis of grid-connected PV is on the built environment, also known as

building-integrated PV (BIPV). Most often, PV installations are part of the existing

infrastructure, or are integrated into the building structure of residential, office or

industrial buildings. Roof-mounted PV systems, for example, are considered a

building-integrated application. In most applications, the electrical power

generated by solar energy is fed into the internal electrical grid of the building.

PV technology

The three main components of a PV system (see figure 3) are the PV cells and

panels (A), the inverter (B), and the meter that records the amount of power

produced (C). For PV systems without a grid connection (D) – so called stand-

alone PV – batteries (E) are also a necessary component.




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Figure 3: PV system overview [3]

Photovoltaic cells

Most commonly, photovoltaic cells are produced from monocrystalline or

multicrystalline silicon material. The efficiency of monocrystalline cells issignificantly greater than that of multicrystalline or polycrystalline silicon.

Monocrystalline silicon is produced as single crystal ingots, while multicrystalline

manufacturing starts with melting the material, followed by a solidification

process with a predetermined crystal orientation structure, resulting in

multicrystalline blocks.

A

B

C

D

E



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Table 1: Technology for PV cells

To produce PV cells, the silicon ingots or blocks are sliced into thin wafers.Typically, crystalline cells measure 10x10 or 12,5x12,5 cm2. The colour of

multicrystalline silicon cells is steel blue, while monocrystalline silicon is

anthracite in colour. On top of the cells, a screen of aluminium conductors is

installed.

Photovoltaic panels

A PV module is the basic building block of any PV power system. A PV module

consists of interconnected cells sealed between a glass cover and weatherproof

backing. The modules are typically encased in frames suitable for mounting. A

PV module contains a series of between 48 and 72 connected cells, Typical PV

modules are 0,8 x 1,2 and 0,8 x 1,6 m2, which corresponds to approximately 80

to 150 Wp, and the average weight of a PV module is approximately 12 kg/m 2.

Two or more modules can be pre-wired together to be installed as a single unit

called a PV or solar panel. Additional PV panels can be added as electricity-

production needs increase. The entire PV system, consisting of one or more

panels, is known as an array.

Inverter

The PV cells and modules generate direct current (DC). Since most household

appliances use alternating current (AC), an inverter is used to convert the DC

voltage to AC voltage, matching the frequency and voltage of the local grid.

Inverters for PV applications include control functions to optimize the power

output, which is referred to as maximum power point tracking (MPPT). The

power output is equal to the voltage multiplied by the current (P = V x I), and the

MPPT function continuously adjusts the load impedance to guarantee optimal

power.

Technology Thin Film Crystalline Wafer Amorphous

Silicon

CIS (Copper

Indium

Diselenide)

Multicrystal-

line

Monocrystal-

line

Module effi-

ciency

6-7% 10-11% 12-14% 13-15%

Area re-

quired per

kWp

15 m2 10 m 2 8 m 2 7 m 2




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In the past, a single inverter was applied for an array or complete PV system.

Currently, however, common practice is to install an inverter for each string, or

even to equip each module with its own inverter; a process that is also referred to

as “AC modules”.

To reduce the losses between the PV panels and the inverter, it is recommended

that you place the inverter as close as possible to the PV panels. In addition, be

sure that the inverter is sufficiently cooled and do not place the inverter in direct

sunlight.

Measuring equipment

To ensure that the PV system is working properly, it is recommended that youhave a measure of the PV system output. The meter records the amount of

electricity (kWh) produced by the system. Note that in some installations, a single

meter is used: the reading on the meter decreases when power is being

generated, and increases when power is being consumed. There are, however,

several metering configurations available, each with their respective advantages

and drawbacks. Ultimately, it is up to the local electrical authority as to which

configuration they will approve.

Grid connection

Depending on the size (Wp) of the PV installation, smaller units can be

connected to the grid by plugging it directly into an electrical socket, whereas

larger units can be connected at the meter board where the cables of the public

grid enter the house.

Batteries

PV systems with batteries for storage are particularly suitable in areas in which a

utility power supply is unavailable or in which utility line extensions would be

prohibitively expensive. The ability to store PV-generated electrical energymakes the PV system a reliable source of electric power both day and night, rain

or shine. PV systems with batteries can be designed to power equipment that

requires DC or AC electricity. People who run conventional AC equipment will

add an inverter between the batteries and the load. PV systems with battery

storage are used all around the world to provide electricity for lights, sensors,

recording equipment, switches, appliances, telephones and televisions.



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Design and installation of PV

A major advantage of PV systems is that they can be easily adopted in existing

buildings or homes. PV systems are modular and can be installed anywhere. In

addition, these types of systems produce no noise, harmful emissions or

polluting gases, and most importantly the energy produced is free. Manufacturers

have designed several different models, which can be placed at a variety of

different types of houses or buildings.

Design

PV panels convert the light that reaches them into electricity. The amount ofelectricity they produce is roughly proportional to the intensity and the angle of

the light that reaches them. The panels, therefore, are positioned to take

maximum advantage of available sunlight within the constraints of their

placement. Maximum power is obtained when the panels are able to track the

sun's movements during the day and throughout the various seasons. These

types of panels, referred to as trackers, are usually ground-mounted using a

heavy steel pole sunk into a concrete foundation. Roof-mounted tracking units

are rare, because they can create structural problems and tend to be noisy

during windy weather.

The best elevations for PV systems vary by latitude. The optimal orientation of

the PV modules is due south. If the orientation is not to the south but e.g. to the

South east or South West, output decreases be a few percentage points. The

optimal tilt angle, with respect to the horizontal, is approximately 41° for Northern

Europe, 35° for Central Europe, and about 32° for Southern Europe [4]. The

optimal tilt angle is higher during winter and lower during summer.

As shown in figure 4, maximum solar irradiation values vary between 1.000 W/

m2 for Central and Northern Europe (with the exception of Northern

Scandinavia) and approximately 1.600 up to 1.800 W/m2 for Southern Europe.

Figure 3 also provides an indication of the yearly amount of electricity (kWh)

produced by PV by geographical region. As you can see in the illustration, a

1.000 Wp PV system located in Southern Europe, for example, produces

approximately 1.250 kWh, while a similar system in Northern Europe produces

approximately 750 kWh.




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Figure 4: Electrical output by geographical region [5]

Based on the data provided in figure 4, we can easily calculate the size of the PV

system, depending on the type of cells. The installed power for crystalline silicon

cells is approximately 100 Wp/m2 and 50 Wp/m 2 for thin-film cells. If an PV

installation is required that produces 875 kWh per year – equal to 25 per cent of

the average annual electricity consumption of an European household (3.500kWh) – the size of the installation in Belgium (1.000 kWh/m2) would be

approximately 1.170 Wp, while the size of the installation in Italy (1.600 kWh/m2)

would be approximately 730 Wp. Depending on the type of cells, the size

required in Belgium is approximately 11,7 m2 (silicon) and 23,4 m 2 (thin film), for

Italy, the size required is approximate 7,3 m2 (silicon) and 14,6 m 2 (thin film).

Obviously, the investment required for a PV installation with similar electricity

production in Italy will be lower than the installation in Belgium.



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As shown in figure 4, a standard performance ratio of 75 per cent is used. Along

with geographical location, the performance of the system output is also be

affected by factors such as the following [6]:

Shading: One of the main factors in the design and placement of a new PV

system is that it be free from obstacles that cause shading on part of the PV

system. Trees, chimneys and other roof protrusions, for example, are well known

obstacles that can lead to shading losses on roof-mounted PV systems.

The problem is that shaded PV cells act like a strong resistor, dissipating the

electricity generated by solar units to the remaining, non-shaded, area of the

string. This is observable through the high temperature (hot spot) in the shaded

modules of a partly shaded system. Frequent high-temperature cycles shorten

the lifetime of a cell and module. Currently, most module manufacturers supply

their products with bypass diodes to prevent a fully or partly shaded module from

sapping the generated energy of the other string modules.

Standard test conditions: The output of the solar PV system is rated by

manufacturers under standard test conditions. These conditions are easily

recreated in a factory and enable consistent comparisons of products, but

nevertheless need to be modified to estimate output under common outdoor

operating conditions.

Temperature: Module output power reduces as module temperature increases

(0,5% per degree Celsius).

Dirt and dust: Dirt and dust can accumulate on the solar module surface,

blocking the sunlight and reducing output. In regions with heavy annual rainfall,

the problem is mostly avoided because the dirt and dust is cleaned off by rain

showers.

Mismatch and wiring losses: The maximum power output of the total PV array is

always less than the sum of the maximum output of the individual modules. The

difference is the result of slight inconsistencies in performance from one module

to the next, and is known as “module mismatch”. Power is also lost to resistance

in the system wiring.




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DC to AC conversion losses: The DC power generated by the solar module must

be converted into common household AC power by means of an inverter. Some

power is lost in the conversion process, and there is additional loss in the wires

from the modules at the roof down to the inverter.

Residential installations

Photovoltaic modules can either be integrated into roofing materials or mounted

on the ground or pole. Whatever the mounting, the structure should be stable

and durable, and be able to support the modules and withstand wind, rain, hail

and other outdoor conditions.

Figure 5: Building integration of PV and Flat roof PV installation (Netherlands) [6]

PV applications in the built environment, as well as ground-based installations

are manifold each requiring a specific type of integration or support structure. A

wide range of products has been developed for use in PV module installation.

Particularly in the built environment, mounting and support structures are

designed in such a way that the PV system is fully integrated into the building

and contributes to its aesthetic and architectonic value. PV support structures are

available for façades, slanted roofs, flat roofs and “PV tiles” that can be used toreplace conventional roof tiles.

Often, the most appropriate and convenient location to place a PV array is on the

roof of a building. The PV array can be mounted above and parallel to the

surface of the roof with a standoff of several centimetres for cooling purposes. In

some cases, such as with flat roofs, a separate structure with a more optimal tilt

angle is mounted on the roof. When considering a roof-mounted PV installation,

attention must be given to the structure of the roof and weather sealing of the

roof.



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Figure 6: PV on slanted roof (UK) [6] and PV on façade (Sweden) [7]

Operation and maintenance

Operation and maintenance of a PV system is simple and requires no extensive

maintenance or upkeep. PV systems contain no moving parts to wear out, break

down or replace. Operation of the PV system should be checked by measuring

the kWh produced by the system. Depending on the amount of dirt and dust

build-up, the PV panels should be cleaned annually. (In most European

countries, the amount of annual rainfall is sufficient to clean most dirt and dust

from the PV panels). You must also ensure that the PV system remains free of

shading throughout its lifetime; growing trees and new home construction, for

example, can lead to shading on the PV system.

Batteries on PV systems require maintenance. The batteries used in PV systems

are similar to car batteries, but are built somewhat differently to allow more of

their stored energy to be used each day. Batteries designed for PV projects pose

the same risks and demand the same caution in handling and storage as

automotive batteries. Batteries must be protected from extremely cold weather

and the fluid in unsealed batteries must be checked periodically.

Costs and benefits Along with investment costs, an economic evaluation of PV systems includes

other aspects which should also be taken into account:

1. Reduction of the annual electricity costs due to the production of electricity

by the PV system: future expectations of the electricity price should be

taken into account;

2. Possible positive stimulation programs by government for PV systems: for

example, subsidies or tax incentives;

3.

The costs of saving on other building materials through the use of PVmodules;




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4. Costs of CO2 pollution due to the production of electricity: zero for PVsystems.

Investment costs

As of 2007, prices for PV systems are between 5 and 7 euros per Wp (including

taxes). These prices are expected to drop to 3,50 euros per Wp by 2010, and to

2 euros per Wp by 2020 (excluding taxes) [8]. Furthermore, many producers

offer performance warranties of 20-25 years on their modules. Some countries

and grid operators also provide subsidies for purchasing PV systems.

The generation costs of household PV systems are, in most cases, not yet

competitive with residential electricity prices, except where support programs arein place. Electricity prices vary greatly across the 27 EU countries. According to

Eurostat, the average price of electricity for an average household within the EU

(as of January 2007) is approximately €0,1528 kWh [9].

Evaluation of PV systems

To get a quick indication of the generation costs for PV systems in homes, divide

the investment costs of a PV system by the amount of kWh produced during the

lifetime of the PV system. With the PV system as described in chapter 3.1, you

can easily calculate the generation costs per kWh.

The installation with an output of 875 kWh per year (25% of annual

consumption), will produce over a life time of 25 years, an output equal to 875 *

25 = 21.875 kWh. For this, PV system of 1.170 Wp is required in Belgium. For

Italy 730 Wp suffices. If you consider an investment cost of 6 euros per Wp, the

cost of 1 kWh in Belgium is € 0,3209 per kWh, and in Italy the cost is € 0,2002

per kWh (not taking into account the time value of money).

Prices in each of these places is higher as compared to the average electricityprice for households within Europe, however, the electricity price in Italy for

households in January 2007 was € 0,2329 per kWh. With these prices, PV

systems can be economically competitive for Southern European countries.

While PV electricity costs are higher for some countries, the price is likely lower

than what we can expect to pay 20 years from now; the costs of PV systems has

decreased steadily for several years, while the costs of electricity (kWh) has

increased in recent years. Some countries and grid operators offer higher prices

for kWh generated by PV systems and feed this energy back into the grid. It can,

therefore, make sense to sell electricity to the grid.



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One question that often arises is how much PV power is required for an average

home. This depends largely upon three main factors:

1. the maximum investment you can afford;

2. the maximum number of PV modules that can be placed on your roof;

3. the electricity (kWh) you want to produce with a PV system.

Before making an investment in a PV system, it is recommendable that you

decrease your electricity consumption, for example, by using energy-efficient

appliances. The lower your electricity consumption is, the smaller your PV

system can be. Table 2 provides an indication of the costs and space

requirements for the electricity produced by a PV system in Belgium and Italy for

covering 25, 50, 75 and 100 per cent of average annual energy consumption

(3.500 kWh).

Table 2: PV systems dimensions at various production levels

Most PV systems do not have to meet 100 per cent of your home’s energy

requirements. If your financial resources are limited, you can simply start small.

Install a system that meets, for example, 25 per cent of your annual energy use,

or even lower. As the cost of PV systems decline, you can gradually increase

your system’s size. Moreover, this example does not factor in subsidies for

investment in PV or for higher electricity prices for selling electricity onto the grid.

Along with economic evaluation, PV systems also provide additional benefits,

such as:

• space-saving installation: PV is a simple, low-risk technology that can be

installed anywhere where light is available on the roof or façade of a

building;

• increased efficiency of the electrical network: since power is generated close

to the point of use, losses in the electricity grids decrease. This can also

reduce or postpone investment in the grid, for example, during the summer

when the use of air conditioning units in homes goes up. In this way, PV

systems can reduce peak loading in the grids caused by air conditioning;

• lower utility costs: after your initial investment in a PV system, your monthly

electricity bill will go down; sunlight, after all, is free;

• climate protection: PV systems emit zero carbon dioxide during their

operation;

kW p area (m2) Costs (Euro) kWp area (m2) Costs (Euro) kWp area (m2) Costs (Euro) kWp area (m2) Costs (Euro)Belgium 1 11,7 7.000 2,33 23,3 14.000 3,5 35 21.000 4,67 46,7 28.000Italy 0,63 7,3 4.375 1,46 14,6 8.750 2,19 21,9 13.125 2,92 29,2 17.500

100% (3.500 kWh)75% (2.625 kWh)50% (1.750 kWh)25% (875 kWh)




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• security of supply: if you use a back up system (batteries), your PV system

can operate while no electricity is delivered from the electricity grid.

Installation in your home or business

Installing a PV system at your house or business can be incredibly beneficial.

Before you install a PV system, however, be sure to contact organisations in your

area, who can provide you with local information regarding PV use in homes or

business. This chapter provides some steps to assist you in getting your PV

system up and running.

1 Contact your local utility, insurance agent and architectural review board.

Some utilities and some insurance agents do not like customers to install utility-

connected power generation systems. Therefore, it is important to call both your

utility and your home insurance agent to determine whether you can proceed.

Your local utility can also inform you about possible incentives for PV systems.

Also, ask about the possibility of feeding electricity back onto the grid. Your local

utility can also inform you about the possibilities for potential subsidies on

investment and/or feed-in tariffs.

Some insurance companies and agents, however, may be unwilling to insure

your PV system. If you live in a subdivision with regulations that restrict the use

of solar energy systems, you will need to submit your PV system plans to an

architectural review board. If you perform the installation without prior approval

from the board, you may be required to tear down your newly installed system. If

you run into problems with your architectural review board, you might also

consider installing PV roofing as an alternative. PV roofing blends with yourhome’s appearance and reduces the aesthetic concerns often expressed by

review boards.

2 Installation and maintenance

Proper installation and maintenance is essential for maximizing the energy

performance of a small solar electric or photovoltaic (PV) system. When installing

a PV system, there are many factors to consider, including placement, system

size, electrical safety and so on. PV systems are complex. An improperly



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designed system can endanger your home or utility line workers. PV systems

must not be treated as something a “do-it-yourselfer can install over a weekend

with an instruction booklet. Instead, consider hiring an expert to design and

install your PV system. An experienced PV system professional can also help

educate your local utility and answer questions from building inspectors.

3: Design considerations

Remember, most PV systems do not have to meet 100 per cent of your home’s

energy requirements. If your financial resources are limited, you can start small.

Install a system that meets 10 to 20 per cent of your annual energy use. As thecost of PV systems decline and state and federal incentives become available,

you can gradually increase your system’s size to meet a greater percentage of

your energy use.

Most PV manufacturers in Europe are members of the European Photovoltaic

Industry Association (www.epia.org). This website lists a number of PV

manufacturers for each country, including each company’s website.

End Notes

[1] Picture Suntech (L) and Goldbeck (R)

[2] Van der Wekken, T., 2007, Application Note, “Photovoltaic installations”, KEMA

Consulting, www.leonardo-energy.org.

[3] Dass grosse Buch vom Energiesparen, Pabel-Moewig Verlag KG, Rastatt.

[4] Šúri M., Huld T.A., Dunlop E.D.,2005, “geographical and time variability of the solar

electricity generation in Europe”, Ispra.

[5] Šúri M., Huld T.A., Dunlop E.D. Ossenbrink H.A., 2007. Potential of solar electricitygeneration in the European Union member states and candidate countries. Solar Energy,

81, 1295–1305, http://re.jrc.ec.europa.eu/pvgi

[6] Endecon Engineering, CEC, 2001, “a guide to PV system design and installation”

[7] IEA Photovoltaic Power System Programme, 2007, http://www.iea-pvps.org

[8] PV-trac, “A vision for photovoltaic technology for 2030 and beyond”, 2004

[9] www.epp.eurostat.ec.europe.eu , “Electricity prices for EU households and Industrial

consumers on 1 January 2007”

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