21 November 2012
IntroductionThere were some interesting exchanges in one local forum about the use of Freon chillers and peltier chillers for aquatic and reef work. Many experienced aquatic enthusiasts in the forum argued that peltier chillers will not work. Those used peltier chillers before was of the view that such chiller is not suitable for reef tanks and cold-stored the chiller. This article will present the differences between the 2 types of chillers and their applications in aquatic and reef work.
What is a Peltier Chiller?Peltier chiller is often referred to as an electronic chiller because it uses a semiconductor chip that has hot and cold plates.
When there is a current passes through, the chip will transfer heat between the plates for heat to be rejected or produced depending on the polarity of the plates. Peltier chip was discovered by a French Physicist Jean-Charles Peltier in 1834 when he passed a current through to product heat and cold energy from a junction composed of two different materials. The largest Peltier chip from ebay has a power input around 500 watts. This has about 300 watts or 1/10 hp cooling power, good enough just for a 300 litre tank. Larger and more efficient ones are being developed.
What is a Freon Chiller?Freon Chiller uses a compressor to compress gaseous freon into liquid state, then the freon is allowed to be cooled in a condenser for heat to be rejected to the atmosphere. After that, it is allowed to be expanded into an evaporator chamber either by means of an expansion valve or capillary tube. In the process of expanding from liquid to gaseous freon in the evaporator, it absorbs heat from its surrounding. The cycle repeats when the gaseous freon is returned back to the compressor.
What’re the Differences?As far as the quality of cooling is concerned, there is no difference between the two. This is because water from aqua or reef tank will cycle through heat exchangers of the chillers. For freon chillers, titanium tubes are often used as cooling coil inside the evaporator whereas for peltier chillers, polypropylene or stainless steel sections are used for heat transfer. No copper or aluminium materials will be used in the water heat exchanger unit.
Apart from this, peltier chillers are often smaller in size except for the electricity consumption. They are not as efficient as the freon chillers. Also, there is no difference in the inlet and outlet connection because there is no cooling coil used in peltier chillers.
One aqua enthusiast built this peltier chiller for his 20-litre tank.
What're the Pros and Cons For Peltier Chiller?
Pros1. Can be easily DIYed. Multiple units can be cascaded to give the desired cooling;
2. Light weight and smaller in size;
3. Can use for both cooling and heating by just reversing the polarity of the peltier chips;
4. Lower capital cost
5. Easier to maintain; no moving parts except the cooling fan; spares are a plenty;
6. Peltier chip claimed to have a life expectancy of 200,000 hrs
Cons1. Higher running cost ; Freon chiller is 75% more efficient than Peltier chiller;
2. The fans for aluminium heat sinks can be noisy unless a bigger heat sink with less noisy fans or remote cooling can be used;
Design Pitfalls for Peltier Chillers
Computer Heat SinkThey are a plenty in the market and often being used wrongly for Peltier chips. A wrong selection of a smaller size not only affects the cooling performance but also results premature failure of peltier chips. Too big a heat sink will cost money although the chiller will run more efficiently. An economic will have to be worked out for the selection. Water cooling is a better alternative than forced air cooling because more heat can be transferred when properly designed; and also, cooling devices can be remotely located to minimise fan noises.
The following is a good article to learn more about peltier and the heat sink selection. Read here.
Pitfall No 2
Cooling blocksThere are a plenty also from the market but most are designed for computer cpu cooling where clean water are often used. They are either of copper or aluminium type which is not suitable for use in aquatic or reef tanks. They could be corroded or choked up easily after sometime.
It may be more appropriate to use stainless steel channel cut to form a box with both ends sealed for plastic inlet and outlet. Corrugated stainless steel sheets such as the following can be DIYed to stuff inside the box and stick lightly with non-toxic thermal conductive expoxy glue to help heat transfer.
Polypropylene plastic can also be used as alternative. The heat transfer characteristic of polypropylene plastic is about as good as stainless steel.
As stainless steel may also corrode over time, the other alternative is to use titanium tubes either embeded in the holes of solid aluminum block or high thermal heat transfer epoxy which has thermal conductivity comparable to titanium. Multiple passes can be arranged for better heat transfer.
Confusion about Cooling Efficiency and Coefficient of Performance (COP)
Someone claimed that Peltier has only 8% efficiency as compared to Freon chiller which has 50% when he searched the net and might have tumbled into the following Wiki site. The paragraph in the Wiki site read as follows. He argued strongly that no chillers will ever have cooling efficiency higher than 100%.
Thermoelectric junctions are generally only around 5–10% as efficient as the ideal refrigerator (Carnot cycle), compared with 40–60% achieved by conventional compression cycle systems (reverse Rankine systems using compression/expansion). Due to the relatively low efficiency, thermoelectric cooling is generally only used in environments where the solid state nature (no moving parts, maintenance-free, compact size) outweighs pure efficiencyFor cooling and heating purposes, the efficiency of cooling or heating device is often measured in terms of Coefficient of Performance (COP) and not the Carnot cycle efficient. This is because freon, when allowed to be expanded in the evaporator, absorbs free energy from its surrounding and therefore, the efficiency can be more than 100%. Peltier chiller, on the hand, has no such advantage of getting free energy from the surrounding and therefore, its COP is much lower.
COP is often expressed as
COP cooling = Qc/Win
Where Qc is the cooling output and Win is the power input
Normally, freon chiller has a COP around 1.3 whereas Peltier chiller will have a COP around 0.55. Air conditioner has much higher COP around 2 to 2.5 because it has larger and more efficient condenser and evaporator. Commercial chillers would have COP more than 5.0 because they have super large condensers with multiple water passes in the heat exchangers.
Why no Large Peltier Chillers?Peltier chillers are limited to smaller sizes in the market. They usually have cooling power of less than 150 watts or 500 BTU/hr or 1/20 hp. It is not because larger one cannot be constructed or manufactured. It may be because the running cost may become more manageable for smaller sized chillers; moreover, Peltier chiller has a significant cost advantage in this range.
Why Peltier Failed to Work?Some claimed that Peltier chillers not only failed to work but also tank temperature kept rising. There are several reasons; one of which may be there was not enough air cooling for the heat sink when it was placed in the cabinets. The other could be due to over-rating by the manufacturers. Some claimed Peltier chillers to have COP of around 0.75 with small heat sinks.
EconomicsOne can get a 1/20 hp Peltier chiller for about SGD$170/= and will have to spend around SGD$300 for a similar size freon chiller. Assuming the cooling power is around 140 watts, Peltier will need about 250 watts of electricity and Freon chiller around 110 watts. At current electricity tariff of 0.2727 per kwh and assuming 48 starts/stops per day with 10 minutes running and 20 minutes resting, the electricity cost will work out to be SGD16 per month for Peltier and SGD$8 for Freon chiller. With that difference in running cost, the straight payback for using Freon chiller will be around 14 months.
ConclusionPeltier chillers are only good for those who have a smaller budget and not too worried about the higher running cost or for starters who just wanted to try nano reef keeping for a short period. It is suitable for use as a backup chiller.
References How to Service Resun CL650 Aquarium Chillers
Freon vs Peltier Chiller for Reef Keeping
Cut Cooling Bills by 90%
DIY Bed Cooler
Selecting Nano Chillers for Aquarium