Insulation & heat loss of my DIY cooler

My DIY TEC Cooler has an interior volume of 480 cubic inches (6x8x10) and an interior surface area of 376 square inches (2.61 sq ft). It has an exterior volume of 1.55 cubic feet (12x14x16) and an exterior surface area of 1168 square inches (8.11 sq ft).  It generally has 3 layers of 3/4″ poly-iso insulation (R5) plus a small amount of one-part urethane expanding foam (say, R2?) in some areas, for an estimated R 17 insulation value (sorry, I’m using imperial units here as my insulation comes with R values….)

To calculate the amount of heat that will escape from inside my cooler to the outside (the amount of heat loss I need to counteract with the TEC system to maintain a set 34°F temperature on a 77°F day), we need to know the thermal delta between the inside and outside of the fridge.  (I’ll use 34°F for a good refrigeration value, and 77°F for the exterior temperature).

I’ll also use the average value of the interior and exterior surface area ((2.61+8.1) / 2) = 5.36 sq ft for this calculation.

In imperial measurements:
[5.36 * (77-34) ] / 17 = 13.55 BTU/h / 3.41 = 3.97 watts

Using the SI system with things translated appropriately gives similar numbers:
0.49796 (25-1) / 2.99 = 3.99 watts

Of course, the above numbers may be completely incorrect, so I also did an experiment after building the cooler:

At 4:45pm I placed 3 refrigerated 12oz (355ml) cans of generic Dr. Pepper in my homemade DIY cooler with a temperature of 3.8°C.  [The active TEC elements were turned off, as I was just testing the insulation properties.]

At 4:21am the next day (11 hours 36 minutes later, rounded to 11.5 hours hereafter) I opened the cooler and one can, measuring the interior temperature at 14.6°C. [we’ll assume all three cans gained the same amount of heat…I only wanted to drink one can.]

So, 36 fl oz (1065 ml) of (basically) water gained enough energy to raise its temperature 10.8°C in 11.5 hours. The specific heat of water is 4.184 J/g-K. 1065ml = 1065 grams of water, or just about 1 kg. Nice how that works out.

4.184 J/g-C * 1065 g * 10.8 C = 48124.3680 Jules = 48.124 kJ = 0.0134 kWh = 13.4 Watt/hour

13.4 wh / 11.5 h = 1.1652173913 watts of continuous energy loss from my cooler.

You might notice that the calculated 3.9 watts is not equal to the observed 1.16 watts.
The main reason for this is that the interior of my cooler was never at 34 °F. It started at 3.8°C (39°F) and then raised up to 14.6°C (58°F) over 11.5 hours. [Also, the ambient temperature was closer to 71.6°F, so the difference between the interior and the exterior was significantly smaller than in my previous calculations.]

For example:
[5.36 * (71-39) ] / 17 = 10.09 BTU/h / 3.41 = 2.95 watts
[5.36 * (71-58) ] / 17 = 4.099 BTU/h / 3.41 = 1.20 watts

However, the integration of the above numbers over 11.5 hours would still give me more heat loss than I observed. So either my experimental measures had a flaw, or the R value of my cooler is higher than the estimated 17.

However, as the results are of the right order of magnitude (4 watts vs 1.1 watts), I’m happy with my calculations and the experiment, and feel that the 4  watts of cooling power needed to maintain a 34°F interior temperature is a good upper bound on the performance needed by my TEC system to maintain temperature.

Alpicool C15 12 volt car fridge / freezer review

I bought an Alpicool C15 model compressor/refrigerant based car refrigerator/freezer for a project. They also have a C20 model which is exactly the same but has a “bump up” in the lid for 20 liters of capacity. I paid $145 with a coupon, and I’ve seen them retail around $190. Here is my Amazon Affiliate  link: https://amzn.to/2rwJvD2

Quick summary of the review video above:  15L interior capacity, can get the interior down to at least -12C, chills four 12oz cans to 3.5C in 4.5 hours, freezes 2 lbs of water in 12 hours, draws up to 50 watts continuous when the compressor is running (never goes above 150 watts on startup so can be ran by my Ego Nexus Escape). It averages 13 watts draw over a 24 hour period at 0 deg C without opening the lid (310 watt/hrs in a day). When set to -20 deg C it draws an average of 25.4 watts per hour (610 watt/hrs in a day).

The amount of insulation is relatively minimal (especially if you are using it as a freezer) but the internal volume / total surface area is small enough that it really doesn’t take up that much power as a refrigerator. Its all plastic construction and friction fit lid is cheap (reflecting the price I paid) but serviceable.  My unit was damaged in shipping and had some plastic break off the side, but still functioned fine. As I plan on voiding the warranty and most of the exterior case in my project, I didn’t mind, but a normal customer would have shipped it back to Amazon for a replacement.

Video on how to use the control panel (including converting the display to Fahrenheit) is here:

Ego Nexus Escape – What I wish I knew before buying

I bought the Ego Nexus escape because Ego gave me a $25 off coupon and I have a lot of Ego batteries from my yard equiptment. I wouldn’t pay full price for it.

 

The Nexus escape is a  small 150 watt (max) SQUARE WAVE inverter that plugs into the top of Ego Arc Lithium batteries, that also provides 2 USB charging ports. Ego Nexus Escape (PAD1500) Amazon (Affiliate) link: https://amzn.to/2Z6qKBn

Great for charging USB devices, or powering small loads where the square wave power isn’t an issue (motors on fans buzz, but most power supplies such as for laptops, routers, etc work fine). The 150 watt inverter will shut down if the load ever goes above 150 watts, as it has zero surge capability over the listed 150 watts.

It will also go into a “low power shutdown” mode to save power if the draw is less than 4 watts, so it is not suitable for running very small loads unattended.

As a small 150 watt inverter, it is serviceable for most small loads, but I really wish it had either a modified sine wave or true sine wave output to make it compatible with more AC devices. Also, having a 300 watt surge capacity for a second or two would have made it much more usable.

Overall, I think the Ego Nexus Power Station is a much better (if much more expensive) product as it has a true sine wave power output, and a lot more capacity.

Whole house solar system: Solar-thermal pool heater, or electric?

Question: “We’re getting whole house solar soon. We are wondering if we should get an electric or solar pool heater?”

To answer this question, I want to keep my terminology clear. I will use “solar-thermal panels” to refer to the rubber roof-mounted panels that have pool water pumped through them for traditional solar pool heaters. I will use “PV solar panels” to refer to photovoltaic panels that generate electricity directly from sunlight. Any pool heater powered by electricity must be a heat pump pool heater (a electric resistance heating element is incredibly inefficient when compared to a heat pump).

There are a lot of trade-offs involved in this question.

First efficiency: Using the sun’s heat to directly heat pool water (as with pumping pool water through a rubber solar-thermal collector on your roof) is about 80% efficient (80% of sunlight is converted to heat in the water). Commercial PV solar panels are only 18-22% efficient converting sunlight to electricity. If you use a restive water heater, this gives you 18-22% efficiency overall, which is bad. HOWEVER, if you use a heat pump electric pool heater, the heat pump uses that 18-22% electricity to move heat from the air into your pool with a 3-4X advantage, so your overall heating efficiency is actually directly comparable to a solar-thermal water heater! [The same math applies to domestic electric hot water heating…hybrid heat pump water heaters are good, traditional resistance elements are not a very efficient use of electricity.]

Second, cost: A heat pump pool heater costs money. So do the extra PV solar panels to power it (plus the up-sized inverter as your PV solar system is larger). I suspect the extra money will be slightly more than a $5,000 solar-thermal water heater arrangement, but it may not be TOO much more. It all comes down to how many kWh you plan on spending to heat your pool. (A heat pump pool heater takes as much power as a whole house AC, it’s basically an AC unit in reverse…pulling heat from the outside air and putting it into your pool.) So this is not an inconsiderable cost.

A 100,000 BTU heat pump pool heater uses 5,000 watts (5kW) when running, or 5kWh per hour of heating. If you wanted to run your pool heater for 5-6 hours every day, you would need to add 5kWh of PV panels to your roof to offset this electric usage. At $2.40 per watt installed, an extra 5,000 watts of PV panels would cost you $12,000!

However, if you only run the pool heater for a few months out of the year and are on a net-metering arrangement, you need to install fewer PV panels, as PV panels produce power all year round. So if you plan on running your pool heater for 5 hours a day only 4 months out of the year, you could get away with installing only 1/3 as many PV panels, or 1.6kW at a cost of $4,000. [You may also have to buy a $2,500 heat pump pool heater if you don’t have one already.]

Third, complexity: A solar-thermal hot water heater requires extra pipes from your pool pump up to the panels on the roof of your house and has a lot of potential leak points. Extra PV solar panels on your roof and a larger inverter don’t add much to the complexity of a whole house PV solar system, just makes it larger, and PV panels have very minimal maintenance issues in the future. An electric heat pump pool heater does require plumbing into your pool pump, but is physically located with the rest of the pool pump equipment, and doesn’t require water pipes up to the roof. Keeping the “roof system” separated from the pool system by electric wires as opposed to water pipes makes “plumbing” easier.

Fourth, roof space: Do you have enough roof space to support all the energy you want to use? Solar-thermal pool heating panels and PV panels used with an electric heat pump pool heater use roughly the same roof surface area for the same amount of heat in the pool. The PV solar panels cost more than rubber pool solar-thermal heat collectors, but have to be replaced less frequently. Heating a pool takes a lot of energy regardless of if you are doing it via photovoltaic or solar-thermal collection. Check with your solar installer to see if you have enough sun facing roof space for everything you want to do.

Fifth, ease of use / flexibility: An electric pool heater allows you to heat the pool whenever you want (cloudy weather, nights). You may need to pay the utility company for your extra use of electricity in bad solar weather, but you have the option to do that if you want. If you install extra PV solar panels to support an electric pool heater, you have the option to NOT run the pool heater and bank extra electricity, which can be used by any electric appliance in the house or an electric vehicle.

[A solar-thermal pool heater prevents you from spending extra money heating your pool at nights or in cloudy weather, but may slightly increase the power needed to run your pool pump unless you actively switch the roof mounted solar heat collectors out of the system when not needed.]

Of course, the cheapest option is to not heat your pool at all.

Moto X4 power button failure (part 2)

Remember how my Moto x4 had a power button failure after 10 months of ownership? Well, the replacement x4 phone that Motorola shipped me had its power button fail in the same way after only a month of usage. Luckily, I still have a month of warranty coverage, and their customer support representative again waived the $24.99 “Premium” fee to ship me a replacement phone before I ship them back the broken one (with a $200 deposit.)

However, I’m not at all impressed with the hardware quality of the power button on this model. I think I will have to just set the screen timeout to a small number of seconds and stop turning the phone off with the power button. (It already has a swipe to activate feature so you don’t need to use the power button to turn it on…)

DIY Thermo-Electric Cooler prototype

I have been playing around with building a DIY Thermo-Electric cooler. Yes, I know the TEC’s are horribly inefficient when compared to a compressor based refrigerator. And I know you can buy basic TEC micro-fridges for $20-$50 online.  We have a camping van that has a small odd sized hole that doesn’t quite fit any of the commercially available car/van coolers, so I’m investigating building my own. This post will discuss prototype number 3.

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Moto X4 power button failure

I’ve had a Moto X4 phone since November of 2018 (I bought it new from Google Fi) and recently the power button started failing intermittently.  Reading forums on the internet, having power buttons fail on phones is now a “thing”. Apparently phone manufacturers are cheeping out on the physical buttons to save money? Seems like a bad part to have fail.

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A certification mark isn’t everything…

I recently purchased a WorkZone Wall Mount Power Board (with detachable tool holder) as an impulse buy at my local Aldi (WorkZone is the Aldi brand for household tools). I needed more light, I could use a power strip, and I liked the fact that each outlet had its own switch so that I could turn on/off battery chargers depending upon if I was using them or not.
workzone wall mount powerstrip with lights and detachable tool holder

The lights worked well, it included a nice template for placing the screws to match the slots in the back and everything was going well until I tried plugging in one of my battery chargers. The outlets were super stiff on the first plug in (not unusual for new outlets), but then they just started to feel dangerously loose. They didn’t appear to grip the plugs well or make good electrical contact. For a device rated at 1875 watts, this was concerning to me.

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Installing Garage Door Slide Locks

My garage has four doors (two in the front, and two in the back) which gives a lot of cross-ventilation potential, but unfortunately some of the doors had the slide-locks installed incorrectly, such that there was no available slots to lock the doors in a “slightly open” position to let air circulate.  They also only had one lock per door, so I rectified that situation by adding a 2nd slide lock to the other side of each door, and moving a few of the original slide locks so that two of the doors can be locked with a 2″ gap below them.  I spent less than $30 for all four slide locks and a box of self drilling sheet metal screws, so it was a relatively quick and inexpensive improvement.