Enhancing the Economics of
Photovoltaic Power Generation with
Innovative Direct Current Applications
UPEx 02 Photovoltaic Experience Conference
Austin Texas,
November 13, 2002
Prepared by:
Phil Barker, [e-mail]
Dave Crudele, [e-mail]
EPRI PEAC Corporation
EPRI
Acknowledgement
This project is funded by:
This project is funded by:
Illinois Clean Energy Community Foundation (ICECF)
Illinois Clean Energy Community Foundation (ICECF)
Electric Power Research Institute (EPRI)
Electric Power Research Institute (EPRI)
Salt River Project (SRP)
Salt River Project (SRP)
The authors wish to thank these organizations for the
The authors wish to thank these organizations for the
funding support. Also, Bob Romo, the ICECF Project
funding support. Also, Bob Romo, the ICECF Project
Manager, and Terry Peterson, the EPRI Project Manager.
Manager, and Terry Peterson, the EPRI Project Manager.
EPRI
Isn t PV Already DC?
What s New Here?
" The natural form of PV energy is direct current
(DC)
" Direct current PV installations are already
widely used for off-grid applications
" What is new here is we are focusing on the use
of DC photovoltaic energy to directly
supplement, without an inverter, power to loads
at sites that already have an AC utility system
interconnection!
EPRI
Candidates for PV-DC Assisted
Applications
" Adjustable Speed Drives (for mechanical
drive motors and HVAC system)
" Uninterruptible Power Systems (UPS)
" Electric Resistance Heating Applications
" Office equipment
" Lighting Loads
EPRI
PV Supplemented Resistive
Heating Loads
Conventional
AC Power Input
(utility)
PV Array (up to 10 kW)
DC level
Rectifier
Converter
DC Bus (250-600 V)
Resistive Heating Element
EPRI
PV Supplemented Adjustable
Speed Motor Drives
Conventional AC Power
Input (utility)
PV Array
Drive
DC level
Rectifier
Converter
Unit
DC Bus (250-600 V)
Adjustable
Speed Drive
Inverter
Overall
ASD Unit
Motor
EPRI
PV Supplemented Office
Equipment
Conventional AC
Power Input (utility)
PV Array
Rectifier
DC level Converter
DC Bus
DC breaker
Computer with
Compatible Switched
Mode Power Supply
Other Compatible
Office Equipment
Computer Monitor with
Compatible Switched
Mode Power Supply
DC Lighting
Equipment
Computer with
Compatible Switched
Computer with
Mode Power Supply
Compatible Switched
Mode Power Supply
EPRI
PV Assisted UPS
Utility Source
Solar Array
Static Switch
Isolating Device
DC
INVERTER AC
Power
Power
Critical Load
Charge
Regulator
Battery
Storage
EPRI
A 125 kVA Solar Assisted UPS System
Developed by Niagara Mohawk Power Corp.
EPRI
Advantages of DC Approach
" Avoiding the inverter might save up to $1/watt (15-20% of
PV system initial capital cost)
" Potentially more reliable installation (could save on long-
term inverter replacement and repair costs)
" Avoiding inverter losses may result in a 5-10% power
production advantage (further lowering cost)
" Limited utility system interaction (no fault current
contributions, no export of power) will make
interconnection less of an issue than with AC inverter
approach
Overall, up to more than a 25% reduction
Overall, up to more than a 25% reduction
in PV lifecycle cost might be achieved in
in PV lifecycle cost might be achieved in
certain suitable DC power applications
certain suitable DC power applications
EPRI
Disadvantages of DC Approach
" Loss of load diversity and ability to export to
utility system means that the application must be
carefully matched to the load to achieve good
economics
" Standard loads and building power distribution
practices, while many can run on DC, are not
intended or rated for DC  modifications are
required
A poorly chosen DC application could
A poorly chosen DC application could
actually cost more than the AC approach!
actually cost more than the AC approach!
EPRI
Switch Mode Power Supply
+
High
Filtered and
Rectifier
Frequency
Regulated DC
Switcher
and
DC
Output
(Chopping
120 Volt, Filtering
Circuit)
Bridge
60 Hz AC
-
Rectifier
Input
Unregulated
Regulated
Filtered and
and
High
60 Hz AC
Regulated DC
unfiltered DC
Frequency AC
EPRI
DC Test Of Switched Mode Supply
EPRI
Compact Fluorescent Light
Operating on DC
EPRI
SMPS Unit for Dell Laptop
EPRI
Adjustable Speed Drives
" Adjustable Speed
Drives can be
employed to convert
most induction motor
loads to operate on DC
power
Typical small
adjustable speed
drive units
EPRI
Adjustable Speed Drive with
Supplemental PV Power
3 Phase 208 volts
3 Phase Variable
+
60 Hz-AC Input
Frequency Output
VA
VB
Motor
DC
VC
-
6 Pole Bridge
6 Pole Inverter
+
Rectifier
DC Input
SCR Bridge
From PV
System
-
EPRI
Load Performance with DC
" Many office devices, adjustable speed motor
drives and other loads can be operated
successfully with DC power
" Perhaps the main load type that can t be powered
are induction or synchronous motors without drive
units
" Despite success in powering loads with DC, there
are some issues:
 Surge arresters
 Protection (circuit breakers, switches and fusing)
 Circuits that need AC for timing or voltage doubling
functions
EPRI
PV-DC Demonstration Site in Illinois
DuPage Mayor s and Manager s Conference Building
Oak Brook, Illinois
EPRI
Major Loads at Demonstration Site
Electric
Total of 9.65 kW of 120 and 240 volt baseboard
Baseboard
Heating
Two 5-ton Units
Rooftop Electric
One 3-ton Unit
Cooling/Gas
One 1.5-ton Unit
Heating
Interior Fluorescents  4.032 kW
Lighting
Exterior Downlighting (Metal Halide)  about 1.1 kW
Exterior Parking Lot Lights (Metal Halide)  2.625 kW
Office
About 5 kW
EPRI
Equipment
Integrated Gas Heat/Electric
Cool Rooftop HVAC Unit
Diagram Courtesy Carrier Corp.
EPRI
HVAC Demand Versus PV Array Output
15
14
13
12
Load of two 5-Ton
11
Peak Daily Output
HVAC Units
10
of Flat PV Array
9
Power
8
7
(KW)
6
5
4
3
2
1
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month of Year
EPRI
10 kW PV Array
5 kW PV Array
2 kW PV Array
Providing Supplemental DC
Power to Rooftop HVAC Units
PV Array (2 to 5 kW DC)
5 ton Rooftop
5 ton Rooftop
HVAC Unit
HVAC Unit
PV Combiner Box,
Fusing and Lightning
Protection
Adjustable
Adjustable
Speed Drive Unit
Speed Drive Unit
DC Voltage Regulator
Control (DC-to DC
Converter Stage)
AC Power from Utility System
(240 V, three phase, 60 HZ)
EPRI
Integrated Load Approach
PV Array (6 kW DC)
5 ton Rooftop
5 ton Rooftop
HVAC Unit
HVAC Unit
Adjustable Adjustable
PV Combiner Box,
Speed Drive Speed Drive
Fusing and Lightning
Unit Unit
DC Rated
Protection
Contactor
DC Voltage
Regulator Control
(DC-to DC Converter
Stage)
DC Baseboard Heating
Control
Auxiliary Port for
AC Power from
System Load
Future DC Signal
Utility System Dispatch
Office Loads
Controller
EPRI
Project Conclusions and
Next Steps
" Many loads can be supplemented with PV-DC (ASD
units, electric heating, SMPS units, and day use lights)
" Load must be carefully matched to the DC output of the
array to insure good economics. An ideal application
may realize 25% or more PV energy cost saving!
" There will be a decision soon to move forward with an
HVAC unit demonstration project in 2003
" The industry needs DC compatible products that have
two power connection ports (an AC port and DC port),
suitable AC/DC appliance ratings and building
distribution systems with DC compatible protection
EPRI