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Early experiences of the 15 kW NMPC demand-side management photovoltaic project
Solar Cells, 30 (1991) 529-533
529
Early experiences of the 15 kW NMPC demand-side management photovoltaic project B r u c e Bailey, Richard P e r e z , J o h n Doty, Kurt E l s h o l z and Ronald S t e w a r t Associated Weather Services, Inc. 55 Colvin Avenue, Albany, N Y 12205 (U.S.A.)
William Huse Niagara Mohawk Power Corporation, 300 Erie Boulevard West, Syracuse, N Y 13202 (U.S.A.)
(Received October 25, 1990)
Abstract The Niagara Mohawk Power Corporation has begun operation of a photovoltaic (PV) system in upstate New York to study the summer peak load reduction capability of gridconnected PV systems serving commercial buildings. The roof-retrofitted system consists of a 151 m 2 polycrystalline silicon module area rated at 15.4 kW d.c., three one-axis trackers, and a high efficiency power conditioning unit. Preliminary results from the first two months of operation indicate PV system output is at a high fraction of capacity when the building experiences its electrical demand peaks. Ongoing studies are evaluating a cross-section of commercial customer load profiles in terms of the probability of peak demand reduction.
1. I n t r o d u c t i o n The Niagara M o h a w k P o w e r C o r p o r a t i o n (NMPC) h a s d e s i g n e d and installed a roof-retrofitted 15.4 k W d.c. P h o t o v o l t a i c (PV) s y s t e m in u p s t a t e N e w York for t h e p u r p o s e o f evaluating the s u m m e r p e a k load r e d u c t i o n capability o f g r i d - c o n n e c t e d PV s y s t e m s serving c o m m e r c i a l buildings. Theoretically, PV are well suited to d e m a n d - s i d e m a n a g e m e n t (DSM) b e c a u s e the availability o f insolation c o i n c i d e s well with the typical daily electrical d e m a n d c u r v e o f c o m m e r c i a l c u s t o m e r s . F o r a m i n i m u m 1 5 - m o n t h period, the p r o j e c t will s t u d y the p r a c t i c a l a n d t e c h n i c a l a s p e c t s o f PV s y s t e m o p e r a t i o n , including issues dealing with reliability, p o w e r quality, load m a t c h ing, n e e d for storage, a n d m a i n t e n a n c e . Ultimately, the potential benefits o f PV-based DSM systems, b o t h to the utility a n d to the c u s t o m e r , are to be evaluated for v a r i o u s t y p e s of c o m m e r c i a l c u s t o m e r s a n d for o t h e r areas of the NMPC service t e r r i t o r y w h e r e w e a t h e r c o n d i t i o n s m a y v a r y f r o m t h o s e at the p r o j e c t site. This p a p e r p r e s e n t s p r e l i m i n a r y p e r f o r m a n c e results f r o m the first two m o n t h s of o p e r a t i o n , July a n d A u g u s t 1990.
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
530
2. System description The PV system consists of: 70 Mobil Solar R a l 8 0 modules (151 m 2 collector area) having a combined rating of 15.4 kW d.c. at standard test conditions, three single-axis tracking frames (with horizontal N--S axis) each with a SunSeeker tracker drive made by Robbins Engineering, and the new high-efficiency Series 3200 Omnion power conditioning unit (PCU) which features automatic maximum power point tracking. The PCU outputs to an existing 480V three-phase electrical distribution panel within the host building. A dedicated PC-based data acquisition system, which includes a Campbell Scientific 21X datalogger and two multiplexers, polls 50 sensors every 10 s, stores 10 min averages and runs preliminary data validation routines. Measurement parameters include: array output, array t em perat ure and orientation, PCU performance, customer demand, and weather and insolation conditions. A harmonics m e a s ur e m ent system capable of recording up to the 100th harmonic measures inverter voltage and current output for one cycle every 10 min. The host building is the headquarters of the New York State Division of Military and Naval Affairs located adjacent to the Albany County Airport. This state-owned facility is primarily an office building that experiences a Monday-to-Friday midday electrical demand peak. The facility's peak demand is approximately 560 kW c o m p a r e d with a base load of about 325 kW. The PV system capacity was determined prior to building selection and thus represents a scale model of what this particular facility could accept.
3. Early results Acceptance tests were conducted to verify that minimum system specifications were met. The specifications included the following: (a) 15 kW d.c. output (_+5%) at 1000 W m -2 at 25 °C panel t em perat ure (b) total system efficiency near full l o a d > 90% (c) PCU power factor > 0.95 under rated array output conditions and > 0.85 at 25% of rated output (d) total harmonic distortion (THD) for c u r r e n t < 5%; individual harmonic d i s t o r t i o n < 3 % THD v o l t a g e < 3 % ; single frequency distortion< 1% These and other tests (e.g., inverter start-up, stand-by operation, maximum power point tracking) found the system to be in compliance with original specifications. The stability of system and c o m p o n e n t perform ance will be monitored for the duration of the project. PV system availability during the first two months of operation e x c e e d e d 90%. Five Ra30 Mobil solar panels underwent indoor and o u t d o o r perform ance tests at SERI's PV Module Testing and Perform ance Facility. The purpose of testing was to establish benchmark module perform ance characteristics prior to extended exposure tests at the project site. The Ra30 modules, onesixth the size in aperture area of the R a l S 0 modules, are being used for
531
testing purposes because of the logistical constraints p o s e d by the larger R a l 8 0 modules in shipping and indoor testing. Figure 1 illustrates average diurnal system output for July-August in terms of theoretical d.c., measured d.c. and measured a.c. The theoretical d.c. power (top curve) is calculated from measured plane of array insolation and manufacturer's output specifications adjusted for operating panel temperature. The actual d.c. output (middle curve) is measured at the PCU. This curve's lower value relative to the theoretical curve is attributed to several factors, included measurement uncertainty, line losses, module mismatching, and imperfect maximum power point tracking by the PCU. The last factor is found to be the dominant effect for low insolation conditions; Fig. 2 shows that the ratio of measured to theoretical DC deteriorates markedly
NMPC DSM-PV Project
lo .................
3
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:,':;::ii ::~i.ii
.............................................
6 .................. :~';:~:................................................ ~,: ............................... 4
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0
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,¢ /
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,~...
............... ~ .................................................
6
8
10
12
14
16
k \
~ , .....................
18
20
22
Time of Day
Pig. 1. Average PV system power output for July-August 1990 expressed as (1) theoretical d.c. determined from plane of array insolation and adjustments for panel t e m p e r a t u r e (top curve), (2) m e a s u r e d d.c. (middle curve), and (3) m e a s u r e d a.c. (bottom curve). 1.2
a t
0.8
I o
0.6
_
0.4
IOm
• []
% 0.2 0 0
200
400
600
800
1000
1200
1400
Plane of Array Irradlance (W/sq.m)
Fig. 2. Ratios of m e a s u r e d to theoretical d.c. power output b a s e d on 10 rain averages in July 1990.
532
at low insolation levels. The lower curve in Fig. 1 reflects the d.c.-a.c. conversion efficiency of the PCU. Near full load conditions, the PCU efficiency is approximately 93%. The average demand reduction during the day for the July-August period as a result of the PV system is depicted in Fig. 3. The upper curve defines the building's diurnal demand on the utility without the PV system while the lower curve shows building demand with the addition of the PV system sized 10 times larger than the present scale model system. The difference between the two curves represents the average demand reduction value of the PV system. Perhaps of greater interest is PV system performance at the times of the building's peak load. To the user, a DSM PV system offers the benefits of demand charge savings which are a function of the system's output when the peaks occur. A comparison of PV system performance with peak building loads is shown in Fig. 4. PV system output expressed as a fraction of its rated a.c. capacity is shown for the times when the top 5, 10, 25, 50 and 100 building loads (10 min average) occurred in July and August. This analysis shows that PV system output is consistently high when it is most needed. Future analyses will be evaluating PV system output during the utility grid's demand peaks. Figure 4 also illustrates the importance of the selected system rating reference. System availability at peak times is shown in terms of (1) a.c. output v s . estimated system capacity at 30 °C ambient temperature and 1000 W zn -2 insolation, (2) a.c. output v s . estimated a.c. capacity at standard conditions (25 °C panel temperature, 1000 W m - e ) , and (3) a.c. output v s . d.c. rated capacity at standard conditions. For the present type of s um m er peak shaving application, the first rating may be the most appropriate measure because the ambient temperature is around 30 °C when the building's highest peaks occur. 551l,
NMPC PV Proiect
50O
\
"~_~45{1
~ E
400
\
T
350
J
300 25C
0
4
8
12 Time of D~y
16
29
24
Fig. 3. Average building demand reduction f o r J u l y - A u g u s t 1990 due to the addi t i on o f PV
system scaled 10 times larger in capacity than existing system. Upper curve defines building demand on the utility grid without the PV system; lower curve defines building demand with the PV system.
533
I
[]
AC (30C airT)
[]
AC (25C celIT)
[]
DC(25CcellT)
80
7O 6O 50 %
4O 30
20 10 0 5
10 25 50 Number of Highest 10-min. Loads
100
Fig. 4. PV system capacity ratios (%) for three reference ratings during the building's highest 5, 10, 25, 50 and 100 10 min loads in July-August 1990. Reference ratings are (1) a.c. output v s . estimated system a.c. capacity at 30 °C ambient temperature and 1000 W m -2 insolation, (2) a.c. output v s . estimated a.c. capacity at 25 °C panel temperature and 1000 W m -a, and (3) a.c. output v s . 15.4 kW d.c. rated capacity at standard conditions.
4. Future plans The results p r e s e n t e d here are b a s e d o n only the first two m o n t h s of a p l a n n e d 1 5 - m o n t h m o n i t o r i n g p r o g r a m a n d t h u s are preliminary. P e n d i n g analyses will a d d r e s s overall s y s t e m a n d individual c o m p o n e n t p e r f o r m a n c e , m a i n t e n a n c e r e q u i r e m e n t s , p o w e r quality, a n d the probability of p e a k d e m a n d reduction. The value a n d sizing o f s t o r a g e will also be evaluated for m a x i m i z i n g the probability o f e n e r g y availability at p e a k d e m a n d . An actual b a t t e r y s t o r a g e s y s t e m m a y be a d d e d to the project. W e also i n t e n d to validate widely u s e d PV simulation p r o g r a m s s u c h as PVFORM a n d evaluate their suitability for s y s t e m design within N M P C ' s service area. W h e r e a s this p r o j e c t is sited at one individual building, the objective is to evaluate the DSM value of PV s y s t e m s u s e d b y the c o m m e r c i a l c u s t o m e r s e c t o r in general. Therefore, time series load data are being o b t a i n e d f r o m a c r o s s - s e c t i o n of c o m m e r c i a l c u s t o m e r t y p e s a n d g e o g r a p h i c a l a r e a s within N M P C ' s service territory ( c o m p r i s i n g half of New York S t a t e ' s total land area). We intend to evaluate the value of distributed PV s y s t e m s to the utility in t e r m s o f net d e m a n d r e d u c t i o n during p e a k periods.
Acknowledgment This p r o j e c t is s p o n s o r e d b y the Niagara M o h a w k P o w e r C o r p o r a t i o n ( C o n t r a c t No E W 7 3 8 8 9 A B R ) a n d the E m p i r e State Electric P o w e r R e s e a r c h Institute.
529
Early experiences of the 15 kW NMPC demand-side management photovoltaic project B r u c e Bailey, Richard P e r e z , J o h n Doty, Kurt E l s h o l z and Ronald S t e w a r t Associated Weather Services, Inc. 55 Colvin Avenue, Albany, N Y 12205 (U.S.A.)
William Huse Niagara Mohawk Power Corporation, 300 Erie Boulevard West, Syracuse, N Y 13202 (U.S.A.)
(Received October 25, 1990)
Abstract The Niagara Mohawk Power Corporation has begun operation of a photovoltaic (PV) system in upstate New York to study the summer peak load reduction capability of gridconnected PV systems serving commercial buildings. The roof-retrofitted system consists of a 151 m 2 polycrystalline silicon module area rated at 15.4 kW d.c., three one-axis trackers, and a high efficiency power conditioning unit. Preliminary results from the first two months of operation indicate PV system output is at a high fraction of capacity when the building experiences its electrical demand peaks. Ongoing studies are evaluating a cross-section of commercial customer load profiles in terms of the probability of peak demand reduction.
1. I n t r o d u c t i o n The Niagara M o h a w k P o w e r C o r p o r a t i o n (NMPC) h a s d e s i g n e d and installed a roof-retrofitted 15.4 k W d.c. P h o t o v o l t a i c (PV) s y s t e m in u p s t a t e N e w York for t h e p u r p o s e o f evaluating the s u m m e r p e a k load r e d u c t i o n capability o f g r i d - c o n n e c t e d PV s y s t e m s serving c o m m e r c i a l buildings. Theoretically, PV are well suited to d e m a n d - s i d e m a n a g e m e n t (DSM) b e c a u s e the availability o f insolation c o i n c i d e s well with the typical daily electrical d e m a n d c u r v e o f c o m m e r c i a l c u s t o m e r s . F o r a m i n i m u m 1 5 - m o n t h period, the p r o j e c t will s t u d y the p r a c t i c a l a n d t e c h n i c a l a s p e c t s o f PV s y s t e m o p e r a t i o n , including issues dealing with reliability, p o w e r quality, load m a t c h ing, n e e d for storage, a n d m a i n t e n a n c e . Ultimately, the potential benefits o f PV-based DSM systems, b o t h to the utility a n d to the c u s t o m e r , are to be evaluated for v a r i o u s t y p e s of c o m m e r c i a l c u s t o m e r s a n d for o t h e r areas of the NMPC service t e r r i t o r y w h e r e w e a t h e r c o n d i t i o n s m a y v a r y f r o m t h o s e at the p r o j e c t site. This p a p e r p r e s e n t s p r e l i m i n a r y p e r f o r m a n c e results f r o m the first two m o n t h s of o p e r a t i o n , July a n d A u g u s t 1990.
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
530
2. System description The PV system consists of: 70 Mobil Solar R a l 8 0 modules (151 m 2 collector area) having a combined rating of 15.4 kW d.c. at standard test conditions, three single-axis tracking frames (with horizontal N--S axis) each with a SunSeeker tracker drive made by Robbins Engineering, and the new high-efficiency Series 3200 Omnion power conditioning unit (PCU) which features automatic maximum power point tracking. The PCU outputs to an existing 480V three-phase electrical distribution panel within the host building. A dedicated PC-based data acquisition system, which includes a Campbell Scientific 21X datalogger and two multiplexers, polls 50 sensors every 10 s, stores 10 min averages and runs preliminary data validation routines. Measurement parameters include: array output, array t em perat ure and orientation, PCU performance, customer demand, and weather and insolation conditions. A harmonics m e a s ur e m ent system capable of recording up to the 100th harmonic measures inverter voltage and current output for one cycle every 10 min. The host building is the headquarters of the New York State Division of Military and Naval Affairs located adjacent to the Albany County Airport. This state-owned facility is primarily an office building that experiences a Monday-to-Friday midday electrical demand peak. The facility's peak demand is approximately 560 kW c o m p a r e d with a base load of about 325 kW. The PV system capacity was determined prior to building selection and thus represents a scale model of what this particular facility could accept.
3. Early results Acceptance tests were conducted to verify that minimum system specifications were met. The specifications included the following: (a) 15 kW d.c. output (_+5%) at 1000 W m -2 at 25 °C panel t em perat ure (b) total system efficiency near full l o a d > 90% (c) PCU power factor > 0.95 under rated array output conditions and > 0.85 at 25% of rated output (d) total harmonic distortion (THD) for c u r r e n t < 5%; individual harmonic d i s t o r t i o n < 3 % THD v o l t a g e < 3 % ; single frequency distortion< 1% These and other tests (e.g., inverter start-up, stand-by operation, maximum power point tracking) found the system to be in compliance with original specifications. The stability of system and c o m p o n e n t perform ance will be monitored for the duration of the project. PV system availability during the first two months of operation e x c e e d e d 90%. Five Ra30 Mobil solar panels underwent indoor and o u t d o o r perform ance tests at SERI's PV Module Testing and Perform ance Facility. The purpose of testing was to establish benchmark module perform ance characteristics prior to extended exposure tests at the project site. The Ra30 modules, onesixth the size in aperture area of the R a l S 0 modules, are being used for
531
testing purposes because of the logistical constraints p o s e d by the larger R a l 8 0 modules in shipping and indoor testing. Figure 1 illustrates average diurnal system output for July-August in terms of theoretical d.c., measured d.c. and measured a.c. The theoretical d.c. power (top curve) is calculated from measured plane of array insolation and manufacturer's output specifications adjusted for operating panel temperature. The actual d.c. output (middle curve) is measured at the PCU. This curve's lower value relative to the theoretical curve is attributed to several factors, included measurement uncertainty, line losses, module mismatching, and imperfect maximum power point tracking by the PCU. The last factor is found to be the dominant effect for low insolation conditions; Fig. 2 shows that the ratio of measured to theoretical DC deteriorates markedly
NMPC DSM-PV Project
lo .................
3
/:;::.ii,
:,':;::ii ::~i.ii
.............................................
6 .................. :~';:~:................................................ ~,: ............................... 4
.............
¢.; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
,.
o_
,¢ /
",,- ~,:
,~...
............... ~ .................................................
6
8
10
12
14
16
k \
~ , .....................
18
20
22
Time of Day
Pig. 1. Average PV system power output for July-August 1990 expressed as (1) theoretical d.c. determined from plane of array insolation and adjustments for panel t e m p e r a t u r e (top curve), (2) m e a s u r e d d.c. (middle curve), and (3) m e a s u r e d a.c. (bottom curve). 1.2
a t
0.8
I o
0.6
_
0.4
IOm
• []
% 0.2 0 0
200
400
600
800
1000
1200
1400
Plane of Array Irradlance (W/sq.m)
Fig. 2. Ratios of m e a s u r e d to theoretical d.c. power output b a s e d on 10 rain averages in July 1990.
532
at low insolation levels. The lower curve in Fig. 1 reflects the d.c.-a.c. conversion efficiency of the PCU. Near full load conditions, the PCU efficiency is approximately 93%. The average demand reduction during the day for the July-August period as a result of the PV system is depicted in Fig. 3. The upper curve defines the building's diurnal demand on the utility without the PV system while the lower curve shows building demand with the addition of the PV system sized 10 times larger than the present scale model system. The difference between the two curves represents the average demand reduction value of the PV system. Perhaps of greater interest is PV system performance at the times of the building's peak load. To the user, a DSM PV system offers the benefits of demand charge savings which are a function of the system's output when the peaks occur. A comparison of PV system performance with peak building loads is shown in Fig. 4. PV system output expressed as a fraction of its rated a.c. capacity is shown for the times when the top 5, 10, 25, 50 and 100 building loads (10 min average) occurred in July and August. This analysis shows that PV system output is consistently high when it is most needed. Future analyses will be evaluating PV system output during the utility grid's demand peaks. Figure 4 also illustrates the importance of the selected system rating reference. System availability at peak times is shown in terms of (1) a.c. output v s . estimated system capacity at 30 °C ambient temperature and 1000 W zn -2 insolation, (2) a.c. output v s . estimated a.c. capacity at standard conditions (25 °C panel temperature, 1000 W m - e ) , and (3) a.c. output v s . d.c. rated capacity at standard conditions. For the present type of s um m er peak shaving application, the first rating may be the most appropriate measure because the ambient temperature is around 30 °C when the building's highest peaks occur. 551l,
NMPC PV Proiect
50O
\
"~_~45{1
~ E
400
\
T
350
J
300 25C
0
4
8
12 Time of D~y
16
29
24
Fig. 3. Average building demand reduction f o r J u l y - A u g u s t 1990 due to the addi t i on o f PV
system scaled 10 times larger in capacity than existing system. Upper curve defines building demand on the utility grid without the PV system; lower curve defines building demand with the PV system.
533
I
[]
AC (30C airT)
[]
AC (25C celIT)
[]
DC(25CcellT)
80
7O 6O 50 %
4O 30
20 10 0 5
10 25 50 Number of Highest 10-min. Loads
100
Fig. 4. PV system capacity ratios (%) for three reference ratings during the building's highest 5, 10, 25, 50 and 100 10 min loads in July-August 1990. Reference ratings are (1) a.c. output v s . estimated system a.c. capacity at 30 °C ambient temperature and 1000 W m -2 insolation, (2) a.c. output v s . estimated a.c. capacity at 25 °C panel temperature and 1000 W m -a, and (3) a.c. output v s . 15.4 kW d.c. rated capacity at standard conditions.
4. Future plans The results p r e s e n t e d here are b a s e d o n only the first two m o n t h s of a p l a n n e d 1 5 - m o n t h m o n i t o r i n g p r o g r a m a n d t h u s are preliminary. P e n d i n g analyses will a d d r e s s overall s y s t e m a n d individual c o m p o n e n t p e r f o r m a n c e , m a i n t e n a n c e r e q u i r e m e n t s , p o w e r quality, a n d the probability of p e a k d e m a n d reduction. The value a n d sizing o f s t o r a g e will also be evaluated for m a x i m i z i n g the probability o f e n e r g y availability at p e a k d e m a n d . An actual b a t t e r y s t o r a g e s y s t e m m a y be a d d e d to the project. W e also i n t e n d to validate widely u s e d PV simulation p r o g r a m s s u c h as PVFORM a n d evaluate their suitability for s y s t e m design within N M P C ' s service area. W h e r e a s this p r o j e c t is sited at one individual building, the objective is to evaluate the DSM value of PV s y s t e m s u s e d b y the c o m m e r c i a l c u s t o m e r s e c t o r in general. Therefore, time series load data are being o b t a i n e d f r o m a c r o s s - s e c t i o n of c o m m e r c i a l c u s t o m e r t y p e s a n d g e o g r a p h i c a l a r e a s within N M P C ' s service territory ( c o m p r i s i n g half of New York S t a t e ' s total land area). We intend to evaluate the value of distributed PV s y s t e m s to the utility in t e r m s o f net d e m a n d r e d u c t i o n during p e a k periods.
Acknowledgment This p r o j e c t is s p o n s o r e d b y the Niagara M o h a w k P o w e r C o r p o r a t i o n ( C o n t r a c t No E W 7 3 8 8 9 A B R ) a n d the E m p i r e State Electric P o w e r R e s e a r c h Institute.