Battery Load Management with the Nextek Power Module
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 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
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.