a case study of two pv systems

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PV IN PRACTICE: A CASE STUDY OF TWO PV SYSTEMS

INSTALLED ON A DOMESTIC AND AN EDUCATIONAL

BUILDINGS

By

Dr. Siddig. A. Omer, Dr. R. Wilson & Professor S. B. Riffat

Institute of Building Technology, School of The Built Environment,

The University of Nottingham, Nottingham NG7 2RDTel: +44 115 8466141, Fax: +44 115 9513159

E-mail: [email protected]

Abstract: This paper presents a case study of two PV systems, one installed on a

domestic building and the other on an educational building viz. the Eco-Energy House

and the Centre for Renewable Energy at the School of the Built Environment, The

University of Nottingham. The PV array on The Centre for Renewable Energy is a

vertical façade, constructed using thin film amorphous silicon cells. The PV array on the

Eco-Energy House is a roof-integrated system using roof slates incorporating

monocrystalline PV cells. The power generated by the PV system in each building is usedto meet building electricity demand, with any surplus exported to the local grid. The two

systems is currently being monitored as part of a two year DTI/ETSU funded programme

to evaluate their performance and highlight any problems affecting the energy output.

Since the two buildings are significantly different in construction, purpose and

occupancy, it was the aim of the project to investigate the effect of these differences on

the selection and operational performance of the building integrated PV (BIPV) system,

and to bring the lessons learned to a wider audience to encourage greater uptake. The

monitoring results also provide useful information to architects and building services

engineers.



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1 INTRODUCTION AND BACKGROUND

Reductions in fossil fuel based energy consumption in the building sector offer the

potential to make cuts in CO2 emissions. In the short term, part of this reduction could beachieved by improving the thermal performance of existing and future buildings to reduce

energy demand. In the longer term, making use of renewable energy resources, e.g., solar

etc., to generate power, could achieve significant reductions in CO2 emissions.

Integration of photovoltaic technology into the envelope of buildings is one of the most

attractive methods for achieving part of the above goal. Firstly, the distributed system

architecture takes advantages of the distributed nature of sunlight to generate power

locally, hence avoiding electricity distribution losses (Baker, 1999; Hestnes, 1999). In

addition, certain building mounted or integrated system configurations have the potential

to reduce part of the balance of system (BOS) costs. For example, mounting an array on a

sloped roof eliminates the need for ground mounting and structure to elevate the array to

the proper angle. In fully integrated configurations, the PV can actually replace some of

building materials and thereby generate a “credit” towards the price of the PV equal to

the value of the material that has been replaced. This strategy requires design guidelines

and know-how to motivate architects and building services engineers to seek new

techniques both for retrofit and new construction.

The School of the Built Environment at The University of Nottingham has constructed

two renewable energy facilities for this purpose. One of these is the Centre for

Renewable Energy and the other is the Eco-Energy House. Both these buildings were

designed to be thermally efficient and utilize a range of renewable energy systems. The

primary role of these facilities is to assist the evaluation of renewable energy and energy

efficient technologies developed by the School and promote renewable energytechnology transfer.

The Centre for Renewable Energy Building (Figure 1) is a two-storey building and

comprises teaching and research facilities for the School of the Built Environment. The

building is naturally ventilated and makes use of daylight, both via conventional windows



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and through extensive use of light-pipes. Solar thermal collectors are installed to offset

energy requirements for hot water. The photovoltaics are used to meet part of the

building’s electrical demand and to educate about Building Integrated PV (BIPV)

systems.

The Eco-Energy House (Figure 2) is a four bedroom detached dwelling of brick and

block construction. The building is provided with a solar-assisted ventilation stack with

heat recovery, a trombe wall and thermal solar thermal collectors to supply part of the hot

water requirement. The PV contributes towards the building’s energy needs. Since these

two buildings are significantly different in construction and occupancy, the aim of the

project to investigate the effect of these differences on the selection and operational

performance of the building integrated BIPV system, and to encourage greater uptake by

bringing the lessons learned to a wider audience.

2 BUILDING INTEGRATED PV (BIPV) SYSTEMS

2.1 PV- Building Integration Choices

a) The Centre for Renewable Energy

The Centre for Renewable Energy has a total roof area of 160 m2

inclined at 16.5o

to the

horizontal and a wall area of about 50 m2 suitable for solar installations. Half of the roof

space has been allocated for thermal solar collectors, to be used for providing part of the

building’s heating load. The rest of the roof space is available for other solar installations,

including BIPV and field-testing of new solar components.

Options available for PV integrated included, installation of PV on half of the roof space,

integration of PVs to the first floor façade (vertical attachment), and application of PVs to

the low-level roof.

b) PV system on the Eco-Energy House

The Eco-Energy House has a total area of about 92m2 suitable for solar installations on

the west, south and east faces of the roof. These all receive direct beam solar radiation,

though some for only limited periods of the day. Half of this space has been reserved for



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solar thermal collectors, and the rest is available for PV integration, i.e., 16m2

on each

face.

2.2

BIPV Design CriterionIn order to determine which space was most suitable for the BIPV system, a number of

options were considered and evaluated. A number of methods were used to assist in the

selection of appropriate PV systems for integration into each building (Omer, 2000;

ETSU, 1999). These included energy analysis using PVSYST3 software, CAD modelling

for assessment of visual impact in addition to architectural and construction

considerations.

a) BIPV System for The Centre for Renewable Energy

Given one of the stated aims of the Renewable Energy Centre is to demonstrate

renewable technologies, it was important that the eventual solution should be prominent

and allow access for students and visitors to see the installation close up. The roof-

mounted arrays did not satisfy these criteria. Consideration of the aesthetics of the

solution turned out to have a significant influence on the selection of the eventual design

solution (Omer, 2000). A vertical array was the favoured option occupying a prominent

position on the façade (Figure 3), and offering access via the first floor windows to thelow-level roof. Opportunities for integrating the array into the fabric of the building were

limited for this solution. The large eaves overhang from the main roof made it necessary

to pull the array forward, away from the first floor wall to avoid shading. The resulting

need for a frame to support the panels and the lost opportunity for substitution of building

envelope materials with PVs, conspired to make this a more expensive solution. The

space created between the back of the array and the brick skin of the building did,

however, allow for effective ventilation to control module temperatures and ease of

access for inspection of PV array components.

The design of the vertical array was also progressed by the building's architect who attempted

to obtain guidance from a number of the main PV manufacturers to progress the design. At the

time there appeared to be little manufacturers' information available to architects to allow

development of designs beyond the initial stage of sizing and positioning the array. Help was



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obtained from a UK PV manufacturer and this led in the adoption of amorphous silicon (a-Si)

as the collector technology on the Renewable Energy Centre. The lower output per unit area of

amorphous silicon PV is potentially less of an issue for installations on commercial buildings,

where large areas of external envelope are available for BIPV.

The PV array installed on this building has a rated capacity of 952W p and consists of 84

single junction amorphous silicon PV cells in 26 modules. The modules are mounted

vertically on the façade with modules under windows tilted at 58o

to the horizontal

(Figure 4). The array is orientated 30° east of south. A Sunny Boy 1100W inverter is

used to connect the PV system to one of the phases in the building 3-phase mains supply.

The 1100W inverter is slightly oversized relative to the PV array (952 Wp) with an

inverter power ratio (IPR) of 115%. IPRs of 70-90% are conventionally considered

appropriate for installations in the UK (Sick, 1996). Oversizing of the inverter will result

in low operating efficiency during periods of low insolation, and hence impaired system

performance and lower energy yield. The selection of the inverter was based on a need to

match the array open circuit voltage.

Installation specifications for the array are given in the Table 1.

Orientation from the south 30 eastTilt angle 90

o

PV array size 19.9m2

Cell Type a-silicon, (Solapak)

Number of panels 28 (four in series and seven strings in parallel)

Rated power 952Wp

Inverter 1100W Sunny Boy

Table 1 Installation specifications of the PV array on The Centre for Renewable Energy

b) BIPV system for Eco–Energy House

In the case of the Eco-Energy House, it appeared that PV integrated into roof slates

offered the best option with good coverage of the available area and visual appearance

that complemented the finishes on the roof surfaces. It was also felt that installing the

PVs using components with which homeowners are familiar, would assist in convincing



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visitors to the Eco-Energy House of the accessibility and applicability of the technology

to domestic building types. Roof slate (or slate effect) systems are commercially

available based both on thin film and crystalline silicon cells.

An investigation was carried out to assess on which part of the roof the PVs should be

installed (Omer, 2000). Attempts were made to investigate the feasibility of increasing

generation in the mornings and evening by distributing the array over east, south and

west facing roof surfaces to match typical domestic demand profiles (Electricity

Association, 1998). However, the penalty in reduced annual production for this solution,

as compared with a south facing array, was severe and the latter solution was adopted,

i.e., roof integration system (Figure 5) on the south-facing roof using PV slates.

Monocrystalline technology, which has higher conversion efficiency than a-Si, was the

favoured option for the Eco-Energy House because of the limited area available on the

roof.

The PV array installed on this building has a rated capacity of 1568Wp and consists of

132 PV Sunslates, each containing six monocrystalline cells. An 1100W Sunny Boy

inverter is used for this system. The inverter was well sized relative to the PV array (1568

Wp) with an IPR of 70%. Installation specifications for the PV array on the Eco-energyHouse are given in the Table 2

Orientation from the south Due south

Tilt angle 52o

PV array size 15.9m2

Cell Type Monocrystalline

Number of Sunslates 132 (two strings of 66 Sunslates)

Rated power 1580 Wp

Inverter 1100 W Sunny Boy

Table 2 Installation specifications of the PV array on The Eco-Energy House

Waterproofing of the PV roof includes overlapping of the rows of slates and the use of

roofing felt beneath. The felt’s close proximity to the rear of the roof slates will result in

some elevated temperature operation of the PV although the roof space is ventilated. The



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PV slates used here have been designed to hang on hooks on the roof battens and have

simple electrical interconnections thus enabling easy installation.

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PV Power Generation and Energy Savings

3.1 Predicted PV Power Generation and Energy Savings

In order to determine the contribution that the two PV systems make towards building

energy loads during the design stage, energy production for the two systems was

predicted using PVSYST3 software. The building energy load for the Renewable Energy

Centre was obtained from measured values. Building energy loads for the Eco-Energy

House were estimated from the typical values of domestic buildings, obtained from the

Electricity Association, (1998). It is worth mentioning that, the Eco-energy House wasoccupied in December 2000 by three research staff who work normal office hours in a

different building and return at lunch time and after work. This helps to provide a realistic

load profile for the building.

Looking at the figures in Table 3, a comparison may be made of the two installations. It

may be seen that despite the large area of the PV array installed on the Renewable Energy

Centre, the system on the Eco-Energy House provides an output slightly over three times

greater. Part of this difference will be due to the difference in efficiency between the

crystalline and thin film cells used in the two installations. The normalised production

(the ratio between the annual energy production to the peak rated power of the array)

figures also suggest that there is an additional cause, possibly the less favourable

orientation and tilt of the Renewable Energy Centre array.

The Centre forRenewable Energy

The Eco-EnergyHouse

Estimated annual PV output, (kWh) 293 1009

Normalized production, (kWh/k Wp /year) 308 638Annual building electrical load, (MWh) 41 3.5

Proportion of annual load met by the PV 0.7% 29%

Table 3 PV system predicted energy production and building load for the Centre for

Renewable Energy and the Eco-Energy House



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As may be expected, the annual electrical loads for the two buildings differ significantly,

due in part to the difference in their size. The consumption in the Renewable Energy

Centre is significantly higher suggesting that the difference in the nature of the

occupation in each building also has an effect. This explains for the difference inproportion of annual load met by the PVs in each building.

A comparison of the PV output and the building energy load for the Renewable Energy

Centre is shown in Figures 6 and 7 for June and January respectively. It may be seen that

the pattern of production matches the building energy load fairly well, though only a

small fraction of the energy demand is met. This may be expected as only a small

proportion of the available envelope area has been covered with PV. Because the PV

array on the Centre for Renewable Energy is not facing due south, the generation curves

do not follow the characteristic bell shape.

Figure 6 Predicted PV output and building load of the Centre for Renewable Energy(sunny day in June)

0

500

1000

1500

2000

2500

3000

3500

0 2 4 6 8 10 12 14 16 18 20 22

Time of the day (hrs)

E n e r g y ( W )

PV output

Building load



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Figure 7 Predicted PV output and building load of the Centre for Renewable Energy

(sunny day in January)

Figures 8 and 9 show the predicted PV output and estimated building loads for the Eco-

Energy House for June and January respectively. These indicate that, in summer, the

array will generate a small energy surplus in the late morning/early afternoon period andmeets a reasonable proportion of the building energy load during daylight hours. The

extra energy will be exported to the grid. As a large proportion of the estimated night

time electricity demand is probably accounted for by lighting using conventional

incandescent bulbs, it is likely that part of this consumption would be reduced through

the use of low energy light bulbs in the Eco-Energy House.

0

2000

4000

6000

8000

10000

12000

0 2 4 6 8 10 12 14 16 18 20 22


E n e r g y ( W )

PV output

Building load



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Figure 8 Predicted PV output and building load of the Eco-Energy House (sunny day

in June)

Figure 9 Predicted PV output and building load of the Eco-Energy House (sunny

day in January)

0

100

200

300

400

500

600

700

800

900

1000

0 2 4 6 8 10 12 14 16 18 20 22


E n e r g y ( W )

Pv output

Building load

0

100

200

300

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500

600

0 2 4 6 8 10 12 14 16 18 20 22


E n e r g y ( W )

PV output

Building load



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3.2 Measured Performance of the PV Systems

a) Performance of the PV system at the Centre for Renewable Energy

The PV system at the Centre for Renewable Energy was installed in March 2000, and

began operation in May 2000. The performance of the PV system has been continuously

monitored since August 2000. Initial predictions using PVSYST software has indicated

that the PV system would deliver about 293kWh per year. However, results from

monitoring of the system performance showed that the PV energy production is

extremely low compared with the predicted values. Figure 10 compares the measured PV

output with predictions.

Figure 10 Comparison between the measured and the predicted PV power generation forthe Centre Renewable Energy System

One of the reasons for this discrepancy is the inverter, which was oversized relative to the

PV array. Analysis of the monitoring data also showed that the inverter has also failing

to track the maximum power point for the array, resulting in extremely poor operation.

0

5

10

15

20

25

30

35

40

45

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

E n e r g y ( k W h )

PV Generation (kWh)

Predicted Power (kWh)

No measurements

for this period



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This has been investigated with assistance from the inverter manufacturer. There is little

experience with grid connecting amorphous silicon PV. This inverter type adopts a

MPPT algorithm which exploits the characteristic of crystalline PV, particularly

operating voltage range and the ratio of peak power voltage to open circuit voltage. Thelatter varies only slightly for crystalline PV with changing insolation levels. The array

voltage for peak power operation of amorphous silicon PV varies considerably as

radiation changes, causing the inverter to fail. The control algorithm of the inverter was

adjusted to allow a wider MPPT voltage range and appears to be giving more reliable

operation. This is, however, an interim solution and is being investigated further to

determine its long-term effectiveness. As a result of this, the inverter manufacturer is

addressing the different operation specifications for amorphous silicon, and is likely to

produce an inverter specifically for this type of PV.

b) Performance of the PV system at the Eco-Energy House

The PV system at the Eco-Energy House was installed in March 2000, and began

operation in June 2000. The performance of the PV system has been monitored

continuously from September 2000. Initial predictions of energy production for the PV

system at the Eco-Energy House, using PVSYST were 1009 kWh per year. However, the

monitoring data showed that, the actual energy production is lower. Figure 11 compares

the measured PV output with the predictions. Although, the inverter for the Eco-Energy

House system is well matched to the PV array for voltage and power, the system has two

constraints that resulted in lower than expected energy production: high temperature and

shading. Restricted ventilation of the array results in high operating temperatures and

lower energy production compared to a freestanding array. An important issue for

crystalline PV is the reduction of the operating temperature through adequate ventilation

to maximise energy production. The Eco-Energy House is close to a number of trees and

the resulting shading results in non-optimal stringing and hence power losses. Work is

underway on a project to increase the void behind the PVs to insure airflow and make use

of the collected waste heat. The School is also in negotiation with the University to trim

some of the nearby trees.



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Figure 10 Comparison between the measured and the predicted PV power generation for

the Eco-energy House System

3. ECONOMIC ANALYSIS OF THE BIPV SYSTEMS

The major cost of the BIPV system is the capital investment required for the installation.

The subsequent running costs are generally low. It is therefore of interest to examine the

distribution of cost between the different system components to investigate those aspects

of the design responsible for incurring the greatest expenditure, in order to inform future

projects. In addition the annualised energy cost for each system was calculated using the

installation costs and assuming a design life of 20 years and an interest rate of 6%. Where

available, measured performance data were used for the electrical production figures with

simulated results from PVSYST being used for those months where data have not yet

been gathered.

0

20

40

60

80

100

120

140

160

180

200

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

E n e r g y ( k W h )

PV generation (kWh)

predicted Power (kWh)

No measurementfor this period



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3.1 Cost of the PV system at the Centre for Renewable Energy

The PV system on Centre for Renewable Energy is one-off demonstration system,

installed for educational purposes. The total capital installation cost of the PV system was

€55,483. This includes the cost of the PV modules, PV array framework, the inverter,

balance of the system, the design and installation costs and value added tax (VAT at

17.5%). Figure 12 shows the cost breakdown for the PV system at the Centre for

Renewable Energy, as percentages of the total investment cost. It can be seen that, as the

system is attached to, rather than integrated into the building, a significant proportion of

the budget is given over to the builders’ work necessary to provide the framework on

which to support the array.

3.2 Cost of the PV system at the Eco-Energy House

Although, the array on the Eco-Energy House was fully building integrated, the supplier,

rather than the builder carried out the installation. There still appears to be some

resistance within trades to tackling new types of work. In this case, the presence of wires

and the requirement to use a screwdriver to make the connections between slates meant

the building contractor’s roofing team were reluctant to attempt the installation. The total

capital installation cost of the PV system was €27,945. This includes the cost of the PV

modules, inverter, balance of the system, the installation cost and VAT. Figure 13 shows

the cost breakdown of the PV system at the Eco-Energy House as percentages of the total

capital cost. As can be seen, a much larger proportion of the Eco-Energy House budget

was available to purchase PV modules, because the system selected was suitable for

integration into a conventional domestic roof structure without the need for any

additional support structure. The labour cost, however, accounted for about half the PV

modules cost. This could have been reduced, if the building contractor had performed the

installation at the same time as installing the remainder of the roof.

3.3 Annualised energy price

The energy costs of the two systems can be evaluated from Table 4, which summarizes

results from the economic analysis of the PV systems at the Centre for Renewable Energy

and the Eco-Energy House. The annualised energy cost of the PV systems on the Centre



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for Renewable Energy and the Eco-Energy House are 22.77€/kWh and 3.12€/kWh

respectively.

The difference in the annual energy costs of the two PV systems is of interest. As the

system at the Renewable Energy Centre is attached to, rather than integrated into the

building, a significant proportion of the budget is given over to the builders’ work

necessary to provide the framework on which to support the array. This added to the cost

of the installation and prevented cost savings being made by substituting conventional

envelope materials with PV modules. If the Renewable Energy Centre system had been

designed to avoid the PV framework, the capital cost of the system would have been

reduced significantly and the annualised energy cost would have been reduced to

11.53€/kWh.

4 CONCLUSIONS

The two PV systems at the School of the Built Environment at Nottingham University are

different examples of building integration and PV technology. The comparison of the two

systems enables the effect of constraints on the design of such systems to be investigated.

This has lead to improved understanding of the behaviour of PV systems, their

integration in buildings, and development of efficient PV system components to

maximise production of PV in the urban environment.

Despite the differences between the two building types, the reason behind the inclusion of

a PV system in the design brief for each was broadly the same i.e., to demonstrate the use

of PV systems in buildings to students and visitors to the University and provide

information to building designers. Differences in the emphasis placed on this

demonstrative role resulted in different approaches being taken when deciding on the

mounting of the arrays.

For the Renewable Energy Centre, it was important that the array created a strong visual

impact so that people were aware the building possessed a PV system. It was also

important that the array is relatively accessible so that it will be possible for students to

inspect the components. The building roof has a large eaves overhang which because of

the inclusion of PV late in the project resulted in a relatively expensive mounting system



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was required. These criteria also resulted in a non-optimal vertical, wall-mounted array

being adopted over a more productive, but less accessible, roof-mounted solutions.

The PV system on the Centre for Renewable Energy has a number of constraints on its

operation including shading in the afternoon, which cannot easily overcome. The use of

amorphous silicon PV is challenging as it has a different characteristic and behaviour

compared to crystalline PV, for which most inverters have been designed.

For the Eco-Energy House it was felt to be important that the array should be non-

intrusive and demonstrate to developers and householders that the technology can be

integrated into a dwelling without impacting on the visual appearance and construction

significantly. This led to the adoption of a roof-mounted solution. The higher output per

unit area of crystalline PV is potentially an important issue for installations on domestic

buildings.

Return on investment was given a low priority in the design brief for both buildings. Of

the two solutions, the Eco-Energy House system benefited from cost savings due to fuller

integration, and therefore offered lower energy cost compared to the low efficiency and

less well-integrated Renewable Energy Centre PV system.

ACKNOWLEDGMENT:

The Authors would to thank ETSU/EDI for funding this project.

REFERENCES

Baker, Pal and Charles Stirling, (1999). Photovoltaics: Integration into buildings. BRE

Scottish Laboratory, CI/SfB 163.

ETSU (1999). Photovoltaics in Buildings, A Design Guide. Report No ETSUS/P2/00282/REP.

Hestnes, Anne Grete, (1999). Building Integration of Solar Energy Systems. Solar Energy,

Vol. 67, Nos (4 – 6), pp. 181-187.



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Electricity Association, (1998). Load Profiles in the 1998 Electricity Supply Market,

Electricity Association. http://www.electricity.org.uk/

Omer, SA, Riffat, SB and Wilson, R, (2000). BIPV design study for Renewable Energy

Centre and Eco-Energy House, ETSU S/P2/00325/REP1.

Sick, S and Erge T. (1996) Photovoltaics in Buildings – A Design Handbook for Architects &

Engineers. James & James.

a case study of two pv systems

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