The use of electrical power to generate heat in consumer products has been in use since the beginning of the electrical distribution grid. The engineering principles underlying this form of heat generation are well established and understood. When the electrical power is obtained from the grid, there is generally little design challenge required to obtain a satisfactory product design.
In contrast, when the electrical power is obtained from a battery, the design challenge needed to obtain satisfactory product characteristics can become extreme. Sometimes the desired product attributes are simply unattainable with the current battery technology.
We will briefly explore some of these challenges and indicate why meeting these challenges using battery power is very unlikely to occur in the near future (5 to 10 years).
For portable heating applications, the battery attributes that are most important in achieving an acceptable product design are:
- Energy Density
- Power Density
- Maximum recommended charge and discharge rate
- Number of Charge/Discharge cycles before battery wears-out
When battery energy density is compared to liquefied petroleum-based fuels, it becomes readily apparent that liquefied petroleum is many times (~ 100x) more than the very best commercially available battery energy sources.
The battery industry has expended a great deal of effort toward increasing the energy density of batteries, especially with Lithium based chemistries. Figure 1 shows the energy density for several different battery chemistries currently in production.
The effort to increase battery energy density has been driven to a large extent by the needs of the EV market. Lithium battery energy density has been increasing slowly over the years, averaging around 5% per year. There are often reports appearing in the engineering journals that suggest that a large increase (i.e. 2X) in energy density is about to happen. Indeed, bench test results for some new designs do support this prediction but the technology never makes it into production because of other issues.
The reason for this is that successful battery production requires that a large number of critical performance and cost parameters be achieved, not just energy density. Dramatically increasing energy density often results in one or more of the other critical battery performance parameters deviating well beyond the acceptable range.
One possible exception (It has as yet to be demonstrated) is the lithium solid-state battery. It has a theoretical energy density improvement of about 2X. Several reputable companies have committed to introducing a prototype EV powered by solid-state lithium batteries before 2025. It is not anticipated that the energy density will increase by the theoretical maximum of 2X, but even if it did, the fuel cartridges that power the Coolfire® catalytic heat products will have about 50x more energy density, at a cost that is many times less.
The practical impact this has on product attributes for small portable heating products can be seen by comparing the CoolFire® catalytically heated travel mug to two battery heated travel mugs, Ember and Cauldron Frye Mobile.
Ember attempted to apply battery power to a travel mug beverage heater while at the same time bounding the size and weight to be within typical travel mug parameters (8” tall with a tapered diameter of 2.8” at the bottom and 3.12” at the beverage cup mouth and a weight of 16 oz.).
Because of current battery technical constraints, the result was that the Ember travel mug does not have sufficient battery energy density to bring a beverage from room temperature to boiling even though the beverage volume is only 12 fl oz. Because of this limitation, it is advertised as a device that maintains an already heated beverage (presumably from a barista) at between 120°F and 140° F for two to three hours. In other words, with current battery technology, it was not possible to store sufficient electrical energy within an acceptable package size to achieve even minimal functionality for an all-purpose cooking, warming, and temperature-maintaining portable travel mug. Even if the energy density were doubled, this would not solve the problem as illustrated in the Cauldron battery-driven travel mug.
The Cauldron travel mug attempts to overcome the performance limitations of the Ember product. It can take a 16 fl. oz. beverage from room temperature to boiling one or two times before a full re-charge of the battery is required. Cauldron’s average time to reach boiling from room temperature is 18 minutes.
To achieve this feat, Cauldron substantially increased its package size and weight. Its height dimension is 50% greater than Ember and its weight is at an astonishing 37 oz (2.3lbs) vs Coolfire’s 14 oz.. Most of the extra weight gain comes from increasing the number of batteries (i.e. increasing the total stored energy available).
As will be explained in the next two sections, Cauldron’s long period of time to boil is NOT because of the energy density limitation of batteries but due to more fundamental limitations that affect all battery heated products.
By comparison, the 16 fl. oz. CoolFire® travel mug has a package size (similar to the Ember travel mug) 8” tall with a tapered diameter of 2.8” at the bottom and 3.75” at the beverage cup mouth with a weight of 14 oz. CoolFire® can perform 12 boiling events with one small 4fl.oz. gas cartridge that fits unobtrusively within the body of the mug.
Because of the ubiquity of electric vehicle technology, Power Density (unlike energy density) is not a parameter that is frequently discussed in trade magazines. This is because energy density relates directly to miles available between vehicle charging.
Nevertheless, for certain portable electric heating applications, Power density can be a more important parameter. This is particularly true for electrically heated travel mugs.
Power (e.g. watts) is the rate at which energy is extracted from the battery. A battery may have a lot of stored energy but if it cannot be extracted at a sufficient rate, it can severely limit its applications. Power density refers to either how compact (volumetric power density) or how heavy (gravimetric power density) a battery will be for a given energy extraction rate (i.e. watts). If you cannot heat the beverage to a given temperature at a reasonable rate, having high energy density doesn’t really help and becomes almost irrelevant. In other words, doubling the energy density has no effect on the time it takes to reach boiling.
This is especially relevant when evaluating potential future battery technologies, such as solid-state batteries, which are projected to have higher energy density than current lithium-ion batteries. This is because they are anticipated to have relatively low power density (e.g. at the low end of current lithium-ion batteries) as shown in figure 1.
Power density limits for batteries are a result of several factors such as internal battery resistance, as well as, potential physical changes that may occur at the anode and cathode which could permanently change its performance. In addition, how well the battery can dissipate internal heat will also affect limits to power density.. Figure 1 is a Ragone plot showing both the energy and power densities for several electrical energy storage devices including Lithium-ion batteries. One useful way to use this chart is to pick any point within the bounds of a particular battery chemistry and note both the energy density and power density. The ratio of Energy density to Power density is the time (in hours) that the battery can operate before being completely discharged.
Achieving the highest theoretical power density may require adding a cooling method which can be either passive (convective and conductive) or active (forced air or liquid). This adds additional weight and/or volume. Active cooling is usually reserved for large applications like electric vehicles. For smaller applications like the battery-driven travel mug, active cooling is not practical which means the actual power density available may be much less than the theoretical.
By comparison, the power density of air/fuel fed catalytic combustion processes are limited only by the air/fuel feed rate which is virtually unlimited (although, from a practical perspective, at some point, very high catalytic power densities may require a redesign of the product to address safety issues). Thus, even a very small fuel cartridge is capable of delivering heat power at levels far beyond a battery of similar size and weight.
In summary, for certain applications, like the heated travel mug or small portable stoves, power density is just as important, if not more so, than energy density and current technology road maps regarding future battery improvements in the next 10 years do not show batteries as a viable energy source to replace or compete with compressed liquid fuel sources in these applications. With the advent of 2nd generation renewable fuel sources, it is believed that the Coolfire® fuel/air driven catalytic heating approach will remain the top contender.
In addition to battery issues of energy and power density, another potential impediment that battery-driven energy sources must contend with is the very nature of electrical power transfer from the battery to the load (i.e. heating coil). A fundamental theorem in electrical engineering (i.e. Maximum Power Transfer Theorem) adds an additional complication to extracting energy from batteries.
ELECTRICAL LOSSES AND INEFFICIENCIES
By way of a simplified design example, let’s say it is desired to heat a 16 fl. oz. volume of beverage from room temperature to boiling in 10 minutes and compare battery and catalytic combustion approaches. A simple calculation shows, assuming no heat loss, that a 16 fl. oz. volume of fluid (water) must receive heat energy at a rate of about 261 watts to reach boiling (100C) from room temperature (i.e. 21C) in 10 minutes.
From figure 1 it is possible to deduce that by pushing the limits of lithium-ion technology, it might be possible to access a power density of 400 W/kg with a maximum run time of 18 minutes before discharging the battery. However, not all of the power can be delivered to the heating coil resistance.
The maximum amount of electrical power that can be delivered to a load (e.g. heating coil) is determined by the Maximum Power Transfer Theorem. The theorem states that maximum power is transferred (red curve in figure 2) to the heating coil when the coil’s electrical resistance (RL) is equal to the internal resistance of the battery (RS). The blue curve in figure 2 shows the battery efficiency. It indicates that the efficiency of the power transfer is 50%. This means that half the power is being lost in the internal resistance of the battery. If the circuit was allowed to operate this way, then for the example given above, the battery would need to deliver a total power of 522 watts. Under this scenario, the battery would weigh at least 1.3 Kg (2.8 pounds) and have a total stored energy capacity of 195Wh (i.e. 150Wh/Kg from Ragone plot multiplied by 1.3Kg). ).
In this case, the battery has to dissipate 261 watts. This can be a very difficult challenge. The very limited space and weight constraints make active cooling impractical and passive cooling (convective & conductive) is not likely to be sufficient.
The only practical approach would be to operate the system at higher efficiencies. Figure 2 illustrates that this moves the operating point further to the right on the blue curve. The trade-off, is of course, that the maximum possible power transfer into the heating coil must be reduced substantially below what is theoretically possible. For instance, if the designer chooses a point on the power transfer curve that is 80% efficient (80% of energy from battery is converted to heat a beverage and the rest is wasted.) the maximum possible power from the battery is reduced to about 64% of the battery’s maximum power density.
The volume occupied by the batteries can be estimated by examining figure 3. The graph shows that the highest volumetric energy density available in current lithium-ion battery chemistries is about 370Wh/Liter. This provides an estimate that the battery pack will occupy about ½ liter of space.
It can be concluded that even setting modest performance requirements (i.e. 10 minutes to boil 16fl.oz.), the battery’s weight and volume are exceeding acceptable levels for a handheld travel mug.
Driving the heater coil for 10 minutes (time to reach boiling from room temperature) will cause 87Wh of energy to be expended during the 10 minute period. This gives the battery the ability to cycle 2.24 times before a recharge is required. In reality it will be less than 2.24 cycles because the power transfer operating point (fig. 2) will probably be between 50% and 80%.efficiency (see below).
To achieve 9 heat cycles on one charge would require 4 times as much battery or 5.2 Kg (11.4 pounds) and occupy a volume of at least 2 liters of volume which is very far from a practical solution for this type of application.
In practice, it is usually desirable to have the system operating at efficiencies higher than the 50%. This is achieved by increasing the load resistance (heating coil) well above the battery’s internal resistance as shown in figure 2 above. Doing this, of course, the percent of total power available to the load is reduced. Generally, the designer requires a specific minimum power transfer without regard for whether one is extracting the maximum power available from the batteries. To achieve this, the number of batteries can be increased in such a way that the net power transferred to the load is increased to the required amount, regardless of the operating point (figure 2).
The disadvantage is that the total number of battery cells must increase well beyond what a casual examination of the battery’s (maximum theoretical) power density might indicate as being sufficient. This makes a bad situation (i.e. weight and package size) even worse.
For an application like electric vehicles, this is an acceptable trade-off because weight and volume increases are more acceptable and high efficiency in EV applications is essential for good mileage.
Contrast this to Coolfire® technology, where a very small (about 4 fl. oz.) and lightweight (3.3oz) fuel cartridge can produce 12 boiling events. Additionally, Coolfire® power densities are not limited by any fundamental properties of the technology as they are in batteries. This means boiling times faster than 10 minutes are obtainable and at a very low fuel cost.