20150212_ESS_NATTBatt_Shaw_Lynds_Hyrbid_Bus

1R. Shaw Lynds, [email protected]

Analysis of Energy Storage Technologies for Hybrid Buses

Feb 2015

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The hybrid buses compared to hybrid autos

Maxwell has over an decade of experience selling Ultracapacitors into the global hybrid bus market. This market is a leader in hybrid vehicle technology in many regards

1. The busses are optimized for a highly urban drive cycle

2. Their drive cycle demands an extremely high power to energy (P/E) ratio from their energy storage system

3. Despite having relatively low stored energy, these buses achieve fuel reductions of between 20% and 30% on average

As hybrid auto market heads towards higher P/E ratio designs to achieve fuel savings with lower mass and cost, the hybrid bus market gives us a window into how different energy storage technologies will be compared in the future

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The hybrid bus energy storage demands

Over the years we have collected several power profiles from our hybrid bus customers, enabling us to compare the power and energy demands

Customer profile

System Discharge Power

System Charge Power

Despite big differences in bus architecture, the power and energy demands overlap nicely for similar sized hybrid busses.

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The hybrid bus energy storage demands

Company Max Power Used

Max Energy Used P/E

178 kW 367 Wh 485 kW/kWh

86 kW 413 Wh 208 kW/kWh

173 kW 422 Wh 410 kW/kWh

20 kW* 1778 Wh* 11.2 kW/kWh

Profile Power and Energy Demands

Customer profile

System Discharge Power

System Charge Power

Note: Max Power is at 50% SOC.

Based on this data, the power P/E ration for hybrid bussed is in the range of 400 kW/kWh.

Compare this to a Toyota Prius with a PE ratio of 11.2 kW/kWh

*Source: 'Thermal Evaluation of Toyota Prius Battery Pack', Matthew Zolot et al, NREL

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Energy storage technologies available

A comparison of commercially available energy storage technologies based on datasheet numbers, clearly shows that UCAPs will be energy limited and Li-Ion will be power limited for the hybrid bus profile

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Simulating their performance in a hybrid bus

Using an energy balanced one hour section of a hybrid bus profiles, we can find the minimum mass of active cell material needed for each energy storage technologies that will meet the power and energy demands of a hybrid bus

This analysis shows that the ACN based UCAP have a dramatic advantage over their PC based counterparts, and are neck and neck with Li-Ion LFP cells for being the lightest solution

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Accounting for the heat generation

However, when we then look at the heating that occurs for each energy storage technology, it becomes clear that the heat generated from cycling such a power and energy optimized pack would pose a significant design challenge

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The practical limit of heat rejection

Heat rejection comes at a high cost at the module level adding both mass and complexity to the system

*Source: 'Thermal Evaluation of Toyota Prius Battery Pack', Matthew Zolot et al, NREL (Active material mass estimated based on Ah capacity)

*Toyota Prius Pack

Maxwell BMOD0063

Internal Prototype #1

Maxwell BMOD0130

Maxwell BMOD0165

While there is no theoretical limit to heat module rejection, a practical limit of 20 W/kg of active material will be used next to compare energy storage technologies

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Comparing technologies based on heat rejection

Re-sizing each system for achieve 20 W/kg of active material, shows that both UCAPs and Li-Ion cells are clearly power and not energy limited.

This simulation shows that when sized for equal specific heat generation, ACN based UCAPs can preform the hybrid bus cycle with nearly half the mass of either PC based UCAPs or Li-Ion batteries

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Summery

As hybrid vehicles designs demand higher and higher P/E ratios from their energy storage systems, Maxwell's unique experience in a hybrid bus market provide insight into the future of vehicle energy storage

1. High heat rejection dominates the design high P/E ratio energy storage systems

2. Comparing energy storage technologies based on power and energy only will not ensure an optimum design solution

3. Technologies must be compared based on leveled heat generation to accurately reflect performance in the hybrid system

ACN based UCAP’s low level of heat generation more then makes up for their relative low specific energy in comparison to Li-Ion batteries in hybrid bus systems