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Round Trip Efficiency & its impact on cost

September 5, 2017

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Inverters and BOP can eat Energy

August 31, 2017

Introduction:   

Battery experts may minimize the importance of the inverter decision and the balance of plant needs but both can impact RTE significantly and therefore the cost of the operation. Consider the cost implications of just 1% Round Trip inefficiency:

For a 200kW Solar – 1MWh system, that can easily cost $1 Million.The effect of each 1% adds $10,000 to the upfront capital cost of the project. As will be seen in this discussion getting the inverter wrong, and not specifying or setting the ancillary equipment right, can quickly add many tens of thousands of the dollars to the overall project investment.

 

The Inverter

Inverters come in all shapes and sizes and some are stand-alone from the charging process and others have these two functions integrated into a single box. Some do not need isolation transformers and for others, this is an essential accessory that steals another 1% away from the overall RTE. Generally, inverters fall into two buckets. Those designed around a nominal voltage of 48V, and those that are many hundreds of volts. Clearly, they serve different types of market but their efficiency curves are similar in shape with a peak efficiency that is typically in the range of 93% to 97%, depending on the manufacturer, but just as much on the generation of system being considered. A typical inverter efficiency chart as a function of power, expressed as a % of full, is shown in the following:

While the peak efficiency may be at 97%, at around 35% of maximum power, it falls off steeply at lower power outputs and there is also a slight decline towards 100% of maximum. At full power the inverter RTE can be several % of its peak. When buying, try and find an inverter where the fall off is minimal between the peak and the full power. One (1) % would be ideal.

 

Inverter Tare, Standby or Idle losses.

I am always suspicious of technical features that attract multiple names and these used to be presented as part of inverter data sheets but have become less visible and possibly without good  reason. Consider the following short assessment looking at two very different, real inverters.

These numbers relate more to standby conditions rather than sleep mode which can be 2/3 less than the standby power drain. The inverter shifting to a lower residual drain mode is not always automatic and not always obvious regarding how to set.

 

HVAC & Ventilation

In earlier parts of this series, polarization losses were discussed and Internal resistance or ohmic polarization was highlighted. This Voltage loss is seen entirely as IR heating and can easily calculated by computing the I(squared)R component. There are few batteries that can easily accommodate heat and this must be managed out of the container, or building, if poor life and balancing problems are not to be experienced. It is important to understand, not just how the IR component drives up the temperature of a system, but also how it changes with time and operating conditions. For example, the Internal Resistance of many batteries can change by a factor of two, depending on the SOC and another factor of two depending on the age of the battery.

Of course IR heating is not the only source of heat in the battery but for the purposes of this blog its effect and the, “cooling energy” required can easily be calculated. Here it is important to note that there are two air cooling methods to manage the heat and each has its advantages and disadvantages. They are described as follows:

i) Simple Ventilation

Air Flow, evenly distributed can be used to maintain the temperature of the battery space, very close to that of the external environment. This method is not suitable for all climates or battery chemistries. LTO, Vanadium Redox and Nickel Iron would be the most suitable as they have very high cycle life and therefore can accommodate higher operating temperature. Molten Sodium would also do well.

 

ii. Air conditioned cooling (& heating).

Air, again evenly distributed, is cooled using an HVAC unit to ensure that the temperature within the battery space is maintained close to the optimal operating temperature. Depending on the external temperature the air may be recirculated or refreshed. This system is essential for AGM lead acid, where the life can be severely compromised by high heat and is necessary for most lithium ion systems if they are to deliver daily cycling performance for ten years or more.

 

There are many variables here but based on calculations relating to a 40ft x 8ft x 8ft container, containing 1MWh of batteries the energy used for each method as a % of 80% of the 1MWh can be calculated. The results of one such analysis is shown in the following table. The best case relates to LFP and the worst case to aged lead acid. For the HVAC example, the energy needed to keep the system temperature constant is presented. For the ventilation model the energy required to ventilate the system and keep the temperature within a few degrees C of its starting temperature. Both cases are ideal but at least illustrative. Reducing the energy for these cooling activities is very possible but tradeoff decisions will have to  be made. IR heating for both C/4 and C continuous rate operations are considered.

As can be seen from the above, the equipment required to manage the temperature of a battery system, can be quite significant andthis is considering the IR component of heating only. Depending on the battery chemistry, other sources of heat can also come into play. For example, the aqueous chemistries will, for sealed systems, e.g. Lead Acid AGM, and somewhat for flooded, lead to the exothermic recombination of oxygen and hydrogen. Aqueous systems, especially flooded, if they vent hydrogen and oxygen in operation, also need to be vented to the outside and this can compromise HVAC routines.

 

Cell to cell connectors.

This needs to be minimized by appropriate design and selection of materials or it can detract from optimum performance. Loss of Voltage in the string, if the resistance is too high, and even more heat must not be underestimated. There are many documents offering information here and many equipment manufacturers will provide guidance. In general, there should be no reason why, at a C/4 rate, this should contribute any more than 0.25% and that is for a 1.2V cell building block. For Lithium Ion with a nominal Voltage of 3.7V, 1/3 the number of connectors are required so the connector-connector resistance should not be greater than 0.1%. For high rate applications copper conductors are a must to maintain this level of loss when the application is high rate. 

 

Battery Balancing (passive)

Capacity matching used to be a very manual and a hit or miss process. It is essential for smooth and high performance of a system to have cells matched in capacity, at the application charge and discharge rates. Balancing works by discharging the strong cells in the string towards the weakest cells. If the spread in capacity is 0.5%, then on a simple basis 0.25% Energy will be lost. If it is 5% a staggering 2.5% will be lost. Based on the author’s experience this can be the range of capacity mismatching experienced in the market at the extreme.

 

Gigavac (or similar) High Voltage Safety protection

This should never be an option for any system above 66V. Switching a high voltage system to smaller, e.g. 48V, safer sub systems should be a matter of course whenever a person approaches within 10 feet of a High Voltage battery system.

Energy will be consumed in something like the Gigavac device from two sources

  1. The Energy needed to power the coil and keep the contactor closed. For a GX23, 12-800V, DC 350 Amp contractor, powered by a 24V power supply, 2.3W is needed to power the coil and keep the contactors shut. For a system with 70 devices, this over a 24 hour period, accounts for just shy of 0.5% of the available energy.

  2. When closed the contactors have a small resistance. It is specified as typically 0.15 to 0.3 mohms. For a 480V system with a 10 series, 7 parallel, configuration, of the same contactors, the current going through each one during a 250kW charge or discharge is 75Amps. The heat lost in each is, I(squared)R or between 0.84 and 1.68W. This loss is only experienced when charging and discharging, 8 hours in total, in this example, and the Energy used up = 472 – 945Wh or between 0.06 and 0.12% of the Energy Storage. This is increased of course for a C rate versus a C/4 rate by a factor of 4. 

In total the Energy consumed by High Voltage switching protectors should not reduce the overall system RTE by more than 1.0%.

 

Other Accessories

The following accessories are strongly recommended for any high performing Energy Storage System.

  1. A system site controller with remote access to data and control

  2. For aqueous systems hydrogen monitoring with feedback systems to terminate operations if LOLs are exceeded.

  3. Lights, security and door locks.

  4. A DC power supply as needed by accessories such as Gigavacs or Hydrogen detectors.

  5. Wifi and Ethernet.

These accessories should not exceed 0.25% of the efficiency in total.

 

Summary

Based on the information presented in the above sub sections a summary table can be presented. 

Corollary 

This lengthy discussion on inefficiency has not touched on the balance of plant required for Molten Sodium or Flow Batteries.  Each of these have their own unique BOP challenges and there is not capacity to review them in this article. This may be the subject of a followup Blog or article but anybody considering these systems can contact us at Hålle@Energyblueshelp.com and we will help them as best as we are able.

 

Next Chapter

In the next episode of the RTE series the RTEs will be combined together in
summary picture. The RTE Doctor will make final recommendations. 

 

For help regarding RTE or any other Energy Storage or Renewable Energy topic please do not hesitate to contact us at, Halle@EnergyBluesHelp.com

 

Thank You

 

 

 

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