Another article on energy-efficient cooling was published in the February issue of our favorite magazine for the plastics processing industry – PLASTECHO. Full text and audio version below.
Winking at my lecturers from Kozminski University, I answer: “It depends.”
It depends on what cooling it requires, how much the finished product weighs, in what temperature range we move, how the cold is produced, what is the loss of its transmission … and only finally how much the reader pays for a kilowatt-hour of electricity. I mention the electricity rate as the last criterion because for different cooling systems the difference in electricity consumption is up to 50% – which is more than the increase at the time of writing this article. As it is easy to see, there are many variables. And while it is easy to determine the investment costs of buying a chiller or building a central cooling system, the cost of operating this system, and consequently the cost of cooling the title “brush”, is very difficult to determine. It is often part of a total electricity bill and mixes with the energy used to power machinery, lighting, and more. Of course, the example given is not a rule, but shows a general tendency.
This text is an extension of another article by me, which appeared in one of the previous issues of “Plast Echo”. So let’s return to the analysis of a 500 kW chiller. Let us assume that two devices operate under the same conditions, with full load, all year round. We produce water at a temperature of 10/15 ° C and as a base we consider a device based on a popular synthetic factor (r410a), and compared to r744 (CO2). Both systems cool the water, the circulation of which is provided by an 8 kW pump. Please note that we have lowered the water temperature in relation to the previous article. Both chillers will produce 4,380,000 kWh of cooling for one year. The F-gas machine will consume 853 185 kWh of electricity during this time, and 474,063 kWh of electricity for CO2. This energy will be used by compressors, condenser fans (or gas cooler), pumps and, in a marginal amount, for automation and control. By making simple calculations, the cost of 1 kWh of cooling for the systems under consideration is as follows (1 kWh of electricity = PLN 1): r410a – PLN 0.18 / kWh of cooling, r744 – PLN 0.10 / kWh of cooling – 44% or 379 savings PLN 122 per year.
Free-Cooling – i.e. free cooling – consists in cooling the process water with the use of outside air when the ambient temperature is sufficiently low. In the case of water 10/15 ° C, we will usually have full cooling power for an outside temperature of about + 2 ° C. Although, unfortunately, I often see dry coolers with a delta of 12 or more degrees Celsius. Due to the risk of water freezing, Free-Cooling often works with glycol with an additional exchanger, which reduces its efficiency. In such a system, the operating time of the compressors will decrease due to the extended operation of the fans, additional pumps and losses on the exchanger. The F-gas system will consume 753 226 kWh of electricity, and the CO2 one will consume 444 458 kWh. The complete data is presented in Table 1.
As we can see, depending on the method of production of cold, it can cost us from PLN 0.09 to over PLN 0.18 for each kWh, and even more in the case of used cooling installations. Maintenance is not included in this value. Since we have established the unit cost of cold, let’s ask how much is needed to cool the title “brush”? In the process of plastics processing, the produced detail, oil and mold are cooled. There is also a loss of coldness in its transmission. In simpler words, it means undesirable heating of cooling water in pipes, pumps and tanks. For the purposes of this article, I made theoretical calculations and consulted the manufacturers of injection molding machines. I should emphasize that the required cooling capacity will vary significantly depending on the material, production temperatures and many factors that the reader probably knows very well. Therefore, further data should be treated more as a description of a specific case than universal knowledge. As a good practice it is assumed that in the production using cooled molds, the chiller should have 50% of the electric power of the injection molding machine, and it is from this value that I used for further calculations. Producing 4 brushes weighing 90 grams per minute, 21.6 kilograms of material will be processed in an hour. Such a machine requires a 20 kW chiller. This gives 0.926 kWh cooling / kg or 0.083 kWh cooling per brush. The final cost of cooling in various configurations is presented in Table 2.
To sum up: the annual average cooling of the title brush costs between 0.8 and 1.5 groszy for each piece of the finished product. In summer, it will be closer to 2 to 4 groszy, while in winter, the cost will be less than the above-mentioned values. Taking into account the unit cost, the aforementioned value is not large, but it is worth paying attention to the graph showing the operation of the mentioned devices on a 15-year scale – and this is (at least) that the chiller should work for.
So we are talking about the potential savings (or unnecessarily spent money) at the level of over PLN 6.1 million, assuming that the price of energy will not increase. This value is many times greater than the additional cost of better refrigeration technologies.
Thermodynamically, cooling is about removing heat. Through the chiller cooling circuit, this heat is finally removed to the environment, i.e. it is treated as waste. And as I will show below – it is a very valuable waste. The considered 500 kW chiller will emit at least 600 kW of heat to the environment (at maximum load conditions), which amounts to 5.25 million kWh per year. This heat obtained from gas at the rate of PLN 0.25 / kWh will be worth PLN 1,314,000 every year. From the “hot side” point of view, it will be a heat pump that can meet the needs of up to 120 homes. A significant difference between the aforementioned chillers is also hidden in ecological issues. However, this is a very broad topic and I will leave it for the next article.
for Plast Echo February 2022
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