Trying a USB Peltier cooler on a tiny moss terrarium: about 2°C cooler for ¥120 a month?

Note: This article is more than one year old.

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I bought a USB Peltier cooler intended for tablets, wondering whether it could cool a small 3.5L moss terrarium to some extent.

I considered building one myself, but without knowledge of power wiring that could be dangerous, so I chose a simple, inexpensive USB unit.

Peltier cooling is weak here; I would not expect it to cool aquarium water.

The cooler was rated at 24W, relatively high for a USB device, and cost about ¥3,500.

Related product listings: Search Amazon for tablet USB Peltier coolers (Ad)

本製品は高性能ペルチェ素子(24W/40*40㎜TEC)を内蔵されたiPhonePromax,Plusやタブレットクーラーです。電源を入れて3秒ほど驚異的な冷却効果を発揮します。
対応機種:タブレット/iPad/iPhone12以降型番Promax,Plus/Switch/Kindle
冷却効果:本機40℃降下
冷却方式:ペルチェ素子
冷却モード:三段階(静音16W;4000RPM/冷却20W;6000RPM/極冷24W;8000RPM)
固定方式:ゴムベルト/磁気吸着式
寸法質量:約109g、約幅72×奥行72×高さ32mm(伸縮ゴムベルトを除く)

Moss in a terrarium apparently does poorly above 30°C, although the species makes a big difference.

https://www.y-michikusa.com/blog/blog/722/

１０～２５℃　適温（気持ちよく成長）
３０～３４℃　成長止まる（弱い種類は痛むものも）
３５℃以上　ダメージ発生の危険

I thought attaching a Peltier cooler to the lid might cool a small 3.5L terrarium reasonably well. Let's test it. The tank measured 15 × 15 × 20 cm.

Related product listings: Search Amazon for small 3.5L tanks (Ad)

The cooler was an unfamiliar Chinese brand sold on Amazon. These products seem to be made entirely in China. I cut a hole in the tank's included cardboard and fitted the cooler. I attached four small aluminum fins to its cold side. Within a few minutes, it became very cold and condensation formed. That could also supply moisture.

Related product listings: Search Amazon for small aluminum heat sinks (Ad)

I placed temperature/humidity meters inside and outside to compare the readings.

Related product listings: Search Amazon for temperature/humidity meters (Ad)

Glass has relatively low thermal conductivity and the lid was cardboard, but I hoped the small tank would cool to some extent.

Test 1: one hour

• Inside the tank:
  • Start: 26.1°C at 15:00
  • After one hour: 24.1°C at 16:00
• Outside the tank:
  • Start: 26.5°C at 15:00
  • After one hour: 27.0°C at 16:00

After an hour, it was 27.0°C outside and 24.1°C inside, apparently a difference of nearly 3°C. The meters differed by 0.5°C at the start; treating that as measurement error and subtracting it gives about 2.5°C. The USB cooler starts automatically when powered on, in its low mode at 15W. The button increases cooling to medium at 20W or high at 25W. A smart adapter can turn power on and off but cannot press the button, so I tested at 15W. The result seemed questionable, so I tried again another day.

Test 2: four hours

• Inside the tank:
  • Start: 26.0°C at 12:00
  • After four hours: 24.8°C at 16:00
• Outside the tank:
  • Start: 27.7°C at 12:00
  • After four hours: 28.5°C at 16:00

The temperature inside stopped falling after about an hour.

My impression was that it cooled by roughly 1–1.5°C.

I could not tell whether this was the Peltier unit's limit or the tank's poor insulation limiting the inside/outside difference.

That is probably about what to expect.

It is a Peltier cooler, after all.

Considering how simple a USB unit is, the result was reasonably good.

It can cool the tank a little, if only for peace of mind.

There also seemed to be a time lag between the inside and outside temperatures. At about 07:30, both read 23°C. The inside appeared to warm about an hour later than the outside.

Test 3: four hours

• Inside the tank:
  • Start: 24.0°C at 12:00
  • After four hours: 23.7°C at 16:00
• Outside the tank:
  • Start: 24.5°C at 12:00
  • After four hours: 26.6°C at 16:00

I still could not tell how much was error, but perhaps the cooler kept it about 2°C below where it would otherwise be. Assuming electricity at ¥27 per kWh, the calculation gives roughly ¥0.4 per hour, or ¥4 for ten hours a day. Very cheap, but was that right? The adapter became extremely hot, which made me wonder.

https://www.denkikeisan.com/?w=15&h=10&m=0&d=1&y=27&s=0

So this USB Peltier cooler seemed able to keep the tank's air about 2°C cooler for ¥4 a day. At ¥120 a month, perhaps that was acceptable. A happy ending.

A few months later

The hot summer continued. A 1–2°C reduction was next to useless. Once summer ended and I could think about it calmly in cooler weather, it seemed unnecessary.

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