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Battery Load Management
with the NEXTEK Power Module
INTRODUCTION
The Nextek power unit offsets electrical load peaks
with the use of its auxiliary battery input. This function
allows power to be diverted from the battery, to the
load, offsetting the AC input peak. The Nextek system
is unique in its ability to reduce hardware costs and
improve battery economics. However, the use of the commercial
battery as a load leveler remains a strong function
of what the electric energy supplier charges for electricity
and the market cost of the storage battery.
The simple question to ask today is "Has anything
changed to improve the value of utilizing battery storage?"
The answer to this is positive with regard to reducing
balance of system cost and improvements in battery economics
as influenced by Nextek's use of direct coupling methods.
Nextek's approach also improves the economics associated
with battery storage by influencing greater cycle life
and power transfer efficiency.
As a historic review of the struggle to make load leveling
an economical alternative illustrates, all previous
approaches have required the use of an AC 60 Hz power
inverter. Since the high cost of the inverter must be
combined with the cost of the storage, the equations
for return on investments have historically been poor.
In addition, other balance of system hardware costs,
such as switchgear connected to the power lines further
aggravated any economic returns. The throughput losses
of the inverter systems just added to the cost. Nextek
has created the tool for incrementally reducing many
cost-based handicapping factors, thus opening the door
for addressing the battery for an economic load shifting
system.
Once a Nextek lighting system is installed, there is
no other balance-of-system cost, other than the battery,
racking system and the external control electronics.
This contrasts sharply with the inverter/battery/charger
based load-shifting methods that consumed energy instead
of conserving it. The projected external controls that
guide the load shift are a small fraction of the overall
cost of implementation and is the focus of this report.
Considering the many factors contributing to the practicality
of load leveling approaches, Nextek’s approach
is better positioned to offset the perplexity of traditional
approaches by reducing hardware costs, coupling losses,
overall interface costs, and thereby the overall installed
cost. In addition, the new regulatory environment, with
its free-market energy pricing has generated interest
in battery load leveling again as well as solar energy
to mitigate daytime power peaks.
Based on the present situation in California, it is
reasonable and prudent to assume that demand costs will
remain high, (and increase). Care must be taken to make
certain that the advantage promised by the Nextek system
is not compromised by the misapplication of the battery
storage. To avoid this, Nextek has carefully studied
battery economics and developed suitable control schemes.
For example, there is a diversity of commercial, lead-acid,
rechargeable storage batteries satisfying a relatively
high value-added market domain. Similarly, not all lead-acid
batteries will prove acceptable for building applications
do to environmental, safety and maintenance issues even
if satisfactory from a cost/performance standpoint.
In this study, a Power Sonic PS-121000, 12 Volt lead-acid
battery was chosen for this benchmark. It has a proven
track record for good performance and a relatively low
cost. It is capable of approximately 85-ampere hour
(AH) of storage at a 17-amp rate. This means it will
deliver 17 amps for approximately 5 hours if deeply
discharged. Two such batteries in parallel would effectively
double the discharge time to the deep cycle limit. However,
it is undesirable to deep cycle a battery since it will
limit its cycle life. The above parallel combination
might best serve a 5 or 6-hour discharge period.
To help put some of the issues in perspective and build
a better understanding, we may configure a load shifting
system using the Nextek's NPS1000 power unit. Such a
system can be expanded indefinitely to accommodate any
size application.
Guided by the capacity limits of the unit it would
support typically a 17 amperes lighting load. Storage
for about 6 hours of sustained load support is eight
12-volt battery units placed in series to satisfy the
nominal 48-volt voltage requirement of the power unit.
The expected cost of these batteries is about $80 each
for a total of $640 per power unit. This bank of batteries
is expected to sustain full lighting for 6 hours from
a fully charged condition. The batteries would occupy
a volume of about four to five cubic feet per power
unit and have at least 500 cycles before replacement
will be necessary.
Management and control conditions will optimize battery
life by:
- Preventing the battery from being used below a
minimum state of charge.
- Not allowing the battery to remain in a discharged
state for extended periods
- Optimizing the charge rate.
This modular system is shown in the layout diagram
below. Each power unit will support 14 twin fluorescent
lamp fixtures at full lighting output. This corresponds
to approximately 1200 square feet of lighted area. Each
power unit will displace approximately 1100 watt of
AC peak during periods when it is operating from the
battery. Given 30,000 square foot building space, this
would correspond to a maximum load shift of (24,000/1000)
X 1100 watts = approximately 26,400 watts or 26.4kW
for up to 6 hours for a total of 158.4 kWh.
Lighted service area 30,000
ft2
Nominal power consumption 24
kW
Maximum load shift as specified 24
kW
Maximum energy displacement per charge 26.4
KWH Max 158.4
Number of power units required 24
Number of 12 volt batteries * required
192
Total cost of the batteries * $15,360
* Many battery combinations can be considered
Nextek power units may be applied to a
simple load management scheme, whereby the interruption
of the AC line to the power unit causes the power to
the lighting to be supported by the storage. This displaces
AC line power, thus mitigating a portion of the AC load
peak. In principle, load peaks can be displaced in proportion
to the number of similar systems in the field. The batteries
are normally maintained at the “float potential”
for long stationary life. The battery is “direct
coupled” to the load with high throughput efficiency.
Nextek’s system does not require a 60 HZ inverter,
therefore “threshold” and throughput losses
are avoided.
THE CONTROL SYSTEM
The micro-controller used by Nextek is a digital
device capable of sophisticated programming and has
become a common solution for low cost control applications.
This device treats information computationally, which
is an essential part of decision making. For example,
should load shifting take place or not. Several kinds
of opportunities exist that might economically justify
load shifting to battery storage. They include:
- Demand charges
- Variable supply pricing
- Time-of-day rates and
- Available-on-demand rates
The controller can determine the shift
in all cases; however, controlling on building-side
load demand will be the most difficult. This is because
demand peaks must be discriminated from a base line
load condition and compared for economic value in a
load shift. This is a difficult task because if the
control point for the shift is too generous, battery
storage will be depleted prematurely with little left
for the major peaks. The result is a demand penalty
at the end of the month and negation of the batteries
system's value.
The described control system is intended as an example
and relates to supply-side variable pricing. Here the
control must make a simple decision between the cost
of storage and the cost of service. If the service price
is higher than the equivalent price of storage reserve,
then the decision will be made to shift to battery until
the price for the electricity drops below the storage
cost per kWh. Another decision will have to be made
as to the price (timing) to charge the batteries.
In this control model, it is assumed that
the real-time service price for electricity is readily
accessible on a real time basis. This may be derived,
for example, from the Internet web site authorized to
display such real time pricing. Other means of inputting
and influencing load-shifting instructions are also
to be considered.
A program is included in the controller to derive the
conditions suitable to commit a load shift. The program
includes other rudimentary functions to assess battery
charge condition and capacity. Battery diagnostics are
also performed with the appropriate algorithm in the
controller.
Nextek uses an AC electronic switch between the AC
line and the Nextek power unit. This will be an ACT
part #RF314. It operates from signals derived directly
from information superimposed on the AC power line.
When this device is signaled to "turn-off,"
the power unit will be isolated from its AC power and
automatically service the lighting load from the battery
bank. Similarly, if the device is signaled to "turn-on",
the power unit will be activated again providing power
to the lighting load and charge to the battery bank.
Code signals from the controller can turn “on”
and “off” selected power units through the
interrupter.
The proposed system will incorporate line carrier communications.
This means that the power lines become the means for
carrying the signal information from the controller.
The controller communicates to the power line through
an interface (ACT product #I103-RS232) that connects
from the serial port on the controller to at least one
power line connection. Complementing this device is
another device called a Coupler Repeater, (ACT product
#CR334) for insuring that all phases of the power wiring
are capable of carrying information.
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