SOLID-STATE POWER SYSTEMS FOR PULSED ELECTRIC FIELD (PEF)PROCESSING.*

M. Gaudreau, T. Hawkey, J. Petry, M. KempkesDiversified Technologies, Inc.

35 Wiggins Avenue, Bedford, MA 01730 USA

Abstract In this paper, DTI will discuss the interrelationshipsbetween pulsed power and PEF processing, and present

Pulsed Electric Field (PEF) processing disinfects liquid examples of system designs and configurations.foods and water. The process kills pathogenic cells byrupturing their cell membranes with short, high voltage I.BACKGROUNDpulses. PEF processing can also improve the performanceof industrial processes such as the removal of water from Traditionally, pasteurization or heat processing is usedsludge, or the extraction of sugars and starches from to reduce the level of bacteria, spores, and other agentsplants, because the ruptured cells release their that cause spoilage of fruit juices, beer, milk, and otherintracellular liquids more easily into their surroundings. liquid foods. Heat processing, however, also reduces theThough PEF systems are based on conventional pulsed flavor of these foods. In PEF processing, an electric field,

power technology, they present a very different set of rather than heat, chemicals, or irradiation, is used to killchallenges in meeting the requirements of specific microorganisms, spores, etc. Kill rates comparable tobiological applications. For example, while most pulsed pasteurization have been demonstrated by multiplepower specifications are based on voltage, current, and researchers across a wide range of liquid foods, includingpower requirements, PEF systems typically start with a juices, milk, liquid eggs, and sauces.desired field strength, flow rate, and range of fluid In PEF processing, a liquid food or other pumpableconductivities. Translating these parameters into a pulsed product, is passed through a small treatment chamber,power system design can be a challenging process. where it is subjected to a short (10 ns - 20 microsecond)

pulse of very high voltage. The high voltage field createdacross the liquid (approximately 35-50 kV/cm) killsmicroorganisms and spores by disrupting cell membranes.The pulses are so short and frequent that all of the liquidin a pipe can be treated as it flows through the treatment

A ~~~~~~~~~~~chamber.By using multiple treatment chambers to applypulses to a stream of fluid, kill ratios of 5-9 log, similar tothose resulting from pasteurization, have been achieved.Multiple experiments have demonstrated that the shelf life

,ZZZ7ZZZZZZ~2 of PEF processed food is comparable to that yielded bypasteurization, with no averse impact on the taste ornutritional value of the food.

Translating these parameters into affordable andeffective PEF systems is a complex problem. The majorfood parameters, such as conductivity, interact directlywith the electrical system design. When the demands of

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II. HIGH VOLTAGE SOLID-STATE load, and is therefore an integral part of the circuit. TheSYSTEMS conductivity of different foods can vary by over an order

of magnitude, depending on the composition of the liquidIn parallel with the development of PEF processing, and the amount of dissolved minerals (e.g., salt) and

Diversified Technologies, Inc. (DTI) has developed solid' solids (e.g., pulp) in the liquid. The conductivity of a

state high voltage pulsed power systems capable of orders single food type, such as orange juice, can vary by 50 -

of magnitude higher performance than conventional 100% due to changes in the raw materials. The PEF

technologies. Solid-state, high voltage systems provide modulator, therefore, must be able to accommodate thesethe reliability and the process consistency required for changes in load impedance. This variability in the load

commercialPEF systems. These benefits enablethe eliminates most transformer coupled modulator designscomrca PE ytm. Ths.eeiseal h from consistent performance in a PEF system. Thetransition of PEF processing from the laboratory into om consiten p ormne i a PEF syste Thcommercial food processing applications. DTI has optimal approach iS to use a 'hard switch', capable of

delivered ~~ ~ .. PEsytm.aal fhadiglwvlm switching the full voltage. This switch must also be lowdelaboratory Fsca temscapableflowratestoflarge,ngpro impedance, to provide consistent output voltage over the

iallations(Fge 2). s, scae wide range of peak currents which may be required as theinstallations (Figure 2). food conductivity varies. Solid-state switches are ideallyThere are three basic elements to the PEF system. First, suited to both of these requirements.

a DC power supply transitions the AC power available The third major element of a PEF system is thefrom the utility into high voltage, DC power. The major telement of the PEF system is the pulse modulator. The. c

requirements for this modulator are, at a general level, applied to the food. The key attribute of the treatment

similar to the requirements of a radar modulator - high chamber is its ability to provide minimal impact on thevoltage and peak power, short pulse, and high frequency. food flow, while ensuring a consistent electric field is

There twocriticldifferecesbetwenPapplied to all elements of the flow. While there are manyThere are two critical differences between PEF and chamber designs, DTI's experience is primarily with the

these RF applications, however. First, a PEF system can co-field flow chamber design, developed and patented bybe designed as either bi-polar (+ and - voltage pulses) or OSU. This design has been shown to provide an optimalmono-polar (all + or all - pulses), and R&D systems balance between the flow and field requirements. Onetypically must provide both bi-polar and mono-polar attribute of this design, however, is that to maintaincapabilities. Radar transmitters are all mono-polar. A consistent field strengths, the gap over which the field iscritical factor in the modulator cost is that a bi-polar applied must be directly proportional to the pipe diameter.system requires four times the switch capability of a Therefore, as larger pipe diameters (to support highermono-polar system of the same voltage and current. Only flow rates) require proportionally higher pulse voltages totwo switches are required, but each switch must stand off maintain the same field strength. This design, therefore, isthe full difference between the positive and negative best utilized at 5 cm pipe diameters and below, whichvoltages when it is open. Second, RF tubes present a translates to 200 kV pulses (at 40 kV/cm) - the currentconsistent and predictable load impedance to the limit for solid-state, hard switched modulators. For largermodulator - at a given voltage, the current required is pipe diameters, alternative modulator or treatmentconstant and predictable. The electrical design of the chamber designs may be required.modulator can be tailored to this specific impedance. InPEF systems, however, the liquid being processed is the A. Key PEF Process Parameters

Any discussion of PEF system design must be basedupon an effective PEF treatment protocol. Developmentof a new protocol requires a significant number of trials atdifferent field strengths and durations, andmicrobiological assessmentof the resulting product. Theliterature contains a great deal of information to guideprotocol development, which can vastly reduce the range

!| iii ...of potential trials required. A typical treatment protocolmight require application of a 35 kV/cm field for a

.1| ~minimum of 50 Fis to achieve a given bacterial reduction._lllll|||lllllllllllllllllllllllllllllllllllllllllllFor the purposes of thispaper,we are assuming that this~~~~~~~~~~~~~~efcc reaquie forPth pariua ppliction

.................... .... . ..... .. .. mm .e discussion Of PE system elements has alreadyv ~~~~~~~~~raisedthe two major factors affecting PEF system design

- food characteristics and flow rate. These two factorsform the basis for all other design tradeoffs. The major

Figure 2. OSU 60kV, 750 A bipolar PEF system. food characteristic of interest is resistivity. This parameterdetermines the impedance of the food in the treatment

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* Bi-polar and mono-polar operation* Fluid Conductivity of 1 to 6 ms/cm* Up to 150 1/hr capability at all conductivities* Four treatment chambers* 20 kW average power

The design process started with the flow rate. DTIdetermined that 150 1/hr could be supported with a 0.3 cmdiameter in the treatment chambers at reasonable pressure(Figure 3). This chamber was approximately 1/3 the sizeof the OSU treatment chambers used in the 2,000 1/hrsystem. This, in turn, determined the peak voltagerequired to be 20 kV, with a peak current ofapproximately 150 A at the highest conductivity. Toprovide these electrical parameters, DTI investigated the

WFU use of a pulse transformer, with a 3 kV, 1000 A primary(which could use an existing, commercial DTI switch).

Figure 3. Cutaway drawing of 20 kV, 150 1/hr Unfortunately, in modeling this design, we found that thePEF treatment chamber. combination of transformer inductance with the

conductivity range made it very difficult to achievechamber, as well as the power (V2/R) required to treat consistent risetimes and pulse shapes. This design wouldeach liter of fluid. The electric field is set by the treatment also require a reset power supply to support mono-polarprotocol, so the energy required to deliver this field to a operation. At that point, we decided to use four hardliter of food is a direct function of the fluid resistivity. switches, at 40 kV each, in an H-bridge configuration, toFlow rate determines several major PEF system provide the pulses.

characteristics. The diameter of the treatment chamber Until recently, simply housing four 40 kV, 150 Amust be sized to pass the desired flow at reasonable switches would have required a very large modulator. Apressure. The presence of particulates and 'chunks; in the key element in this design was DTI's recent efforts toflow can also impact the sizing of the chamber. Since the develop IGBTs specifically for pulsed power applications.gap distance is a function of chamber diameter, and larger ' 4 s ' ' ''gaps~~~~~~~~~~~~ ~reurhihrvlaest,anan, ie il Figure 4 shows one such device, developed ingaps require higher voltages to maintain a given field collaboration between DTI and Powerex, under a U.S.strength, balancing pressure and voltage is a critical Department of Energy SBIR. This device allowed us tooptimization that must be made. build a compact 20 kV, 150+ A switch module, shown inFlow rate also determines the average power required Figure 5. Eight of these modules (2 per switch) were fully

for a given fluid and protocol. The conductivity and field capable of supporting the 20 kV, 150 A bi-polar pulsesstrength required determine the energy per liter required required, and could be assembled in a very compactmultiplying this by the flow rate gives energy/time, which configuration (in oil). These modules are fully capable ofis power. The power required increases linearly with flow supporting operation across the 1 - 10 'ts pulsewidthsrate for a given protocol.rateforagven rotcol required, at frequencies up to several kHz.As one example, at 75 kW average power, the OSU r e, a

system is nominally sized to operate at 2000 liters perhour. For highly resistive foods, such as apple juice, thisunit can process up to 5,000 liters per hour, while highly...........................conductive foods (such as salsa) will have much lower

proessintg, throughpu2t is dirctl p,ropotina to the

avrgUptowe forakgivefield strength .

D1tI reenldepliveewdt anurR&D PEFssystem toniso aPT tp

chronshptrain thereis considerableve.eFort commercial................................lo4boptimize1the2performance8and0costofthisunit,.while

retainingthedesiredflexibilityintreatmentprotocolsthe~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~................................providedbytheuniversityincluded:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.....................................

*Upto60kV/cmfieldstrength Figure4.Pulsepower transistor(PPT)top;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.....................................to 10 ~~~~~~~~~~~~~~~~~~is pulsewidths conventional TO-247bottom.~. ... ... ........................ ....................

IV. FUTURE SYSTEMS

The cost of the system shown in Figure 1 isapproximately $250,000, including the treatmentchambers. While this is consistent with the price of otherpulsed power systems, comparable in power andcomplexity, this is an order of magnitude higher thanconventional food processing systems, such aspasteurization systems. The energy costs for PEFprocessing are typically lower than those of pasteurizationsystems, however, making the PEF process comparable to

Figure 5. 20 kV, 100+ A, PPT switch module. pasteurization over a 2 - 5 year period of time. Reducingthe capital costs of PEF equipment will be an important

The veraepoer as dtermned y te unversty,factor in the commercial adoption of this process.Thesgicaverag cedtepowerwsdtrmin ied

by the univrsIty To achieve this capital cost reduction, DTI isandsinifcatlyexeed te pwe reuied t 50 /h. t developing future PEF systems which are designed to

is, in fact, enough power to boil the fluids at high meet both the microbiological requirements of the PEFconductivities. This power rating offers the ability to process and lower the equipment costs of PEF systems.increase the flow rate of this system to 300 - 400 1/hr, at To achieve these objectives, DTI has identified the40 kV/cm peak field strength, simply by replacing the following design objectives for future PEF systems:treatment chambers. eMonopolar operation, to reduce the number of

Figure 6 is a sample pulse from the configuration switches required,shown in Figure 1. The overall system includes the power *Hard switching (no pulse transformer), tosupply, 20 kV, 150 A H-bridge modulator, and a Pulse accommodate changing product conductivity,Control Unit in a single, 24" rack. The treatment -Simplified power supplies, such as buck regulators,chambers are enclosed in a separate, vegetable oil filled -Smallest treatment chamber gaps possible, to reducetank containing two assemblies, each with a single high peak voltage requirements,voltage input and two treatment chambers (gaps). This *High pulse frequency to accommodate high flow ratessystem provides very fast risetimes (- 200 ns) across the With these design features, we believe that the total sizefull conductivity range. and cost of future PEF systems can be reduced by a factorThe overall system design provides a highly flexible, of two or more, compared to the research systems built to

moderate flow system for a range of PEF R&D and date.protocol developments. This system can also supportcommercial, low volume PEF operations. V. CONCLUSIONS

Multiple researchers have shown PEF processing to beequivalent to pasteurization in terms of pathogenreduction for a wide range of liquid foods. For foods thatare heat sensitive, there are considerable benefits in taste,

* p ~~~~~~~~color,and nutritional value from the non-thermal PEFprocess. The application of PEF to other industrial

........... processes builds directly on the research in foodprocessing, and new applications of PEF are emerging ata significant pace.The use of solid-state, high voltage pulsed power

systems for PEF processing is the key to thesecommercial applications. Solid-state technology allowsthis PEF to scale from smal laboratory systems to large-scale processing facilities. Developing a commonlanguage and process for defining PEF requirements andSystems will help researchers, system developers, andfood processors communicate their needs and constraints,

~~~~ v ~~~~~~~and allow all sides to achieve their objectives moreeconomically. New technologies are allowing PEFsystemto bcome malle and _les xenie.Th ntr