EPRI

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:

Illinois Clean Energy Community Foundation (ICECF)

Electric Power Research Institute (EPRI)

Salt River Project (SRP)

The authors wish to thank these organizations for the
funding support. Also, Bob Romo, the ICECF Project
Manager, and Terry Peterson, the EPRI Project Manager.

This project is funded by:

Illinois Clean Energy Community Foundation (ICECF)

Electric Power Research Institute (EPRI)

Salt River Project (SRP)

The authors wish to thank these organizations for the
funding support. Also, Bob Romo, the ICECF Project
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

PV Array (up to 10 kW)

DC level
Converter

Rectifier

Conventional
AC Power Input
(utility)

DC Bus (250-600 V)

Resistive Heating Element

EPRI

PV Array

DC level
Converter

Drive
Rectifier
Unit

Conventional AC Power
Input (utility)

DC Bus (250-600 V)

Adjustable

Speed Drive

Inverter

Motor

PV Supplemented Adjustable

Speed Motor Drives

Overall
ASD Unit

EPRI

PV Array

DC level Converter

Rectifier

Conventional AC
Power Input (utility)

DC Bus

Computer with
Compatible Switched
Mode Power Supply

Computer Monitor with
Compatible Switched
Mode Power Supply

Other Compatible
Office Equipment

DC Lighting
Equipment

Computer with
Compatible Switched
Mode Power Supply

Computer with
Compatible Switched
Mode Power Supply

DC breaker

PV Supplemented Office

Equipment

EPRI

PV Assisted UPS

Utility Source

INVERTER

Critical Load

Static Switch
Isolating Device

Battery

Storage

Solar Array

DC
Power

AC
Power

Charge

Regulator

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
in PV lifecycle cost might be achieved in
certain suitable DC power applications

Overall, up to more than a 25% reduction
in PV lifecycle cost might be achieved in
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
actually cost more than the AC approach!

A poorly chosen DC application could
actually cost more than the AC approach!

EPRI

120 Volt,

60 Hz AC

Input

Bridge

Rectifier

+

-

Rectifier

and

Filtering

Filtered and

Regulated DC

Output

DC

Regulated

High

Frequency AC

60 Hz AC

Unregulated

and

unfiltered DC

Filtered and

Regulated DC

High

Frequency

Switcher

(Chopping

Circuit)

Switch Mode Power Supply

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

6 Pole Bridge

Rectifier

V

A

V

C

V

B

Motor

6 Pole Inverter

SCR Bridge

3 Phase 208 volts
60 Hz-AC Input

-

+

3 Phase Variable
Frequency Output

DC

DC Input
From PV
System

+

-

Adjustable Speed Drive with

Supplemental PV Power

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

About 5 kW

Office

Equipment

Interior Fluorescents – 4.032 kW

Exterior Downlighting (Metal Halide) – about 1.1 kW

Exterior Parking Lot Lights (Metal Halide) – 2.625 kW

Lighting

Two 5-ton Units

One 3-ton Unit

One 1.5-ton Unit

Rooftop Electric

Cooling/Gas

Heating

Total of 9.65 kW of 120 and 240 volt baseboard

Electric

Baseboard

Heating

EPRI

Integrated Gas Heat/Electric

Cool Rooftop HVAC Unit

Diagram Courtesy Carrier Corp.

EPRI

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov Dec

Power

(KW)

Month of Year

Load of two 5-Ton
HVAC Units

10 kW PV Array

5 kW PV Array

2 kW PV Array

Peak Daily Output
of Flat PV Array

HVAC Demand Versus PV Array Output

EPRI

PV Array (2 to 5 kW DC)

5 ton Rooftop

HVAC Unit

DC Voltage Regulator
Control (DC-to DC
Converter Stage)

5 ton Rooftop

HVAC Unit

Adjustable

Speed Drive Unit

Adjustable

Speed Drive Unit

PV Combiner Box,

Fusing and Lightning

Protection

AC Power from Utility System

(240 V, three phase, 60 HZ)

Providing Supplemental DC

Power to Rooftop HVAC Units

EPRI

PV Array (6 kW DC)

Auxiliary Port for

Future DC

Office Loads

DC Baseboard Heating

5 ton Rooftop

HVAC Unit

System Load

Dispatch

Controller

DC Rated
Contactor

DC Voltage
Regulator Control
(DC-to DC Converter
Stage)

Control
Signal

5 ton Rooftop

HVAC Unit

Adjustable

Speed Drive

Unit

Adjustable

Speed Drive

Unit

PV Combiner Box,

Fusing and Lightning

Protection

AC Power from

Utility System

Integrated Load Approach

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