Basics of Adsorption Technology
Adsorption describes the accumulation of substances (gases or liquids) on the surface of a solid which commonly is known as sorbent. In adsorption chiller systems, this process is designed for cold generation using water/zeolite or water/silica gel as the pairs of working material.
Fahrenheit adsorption chillers use two different types of adsorbents, silica gel and zeolite. Silica gel, which is known for example as a desiccant for electronic devices, is completely non-toxic and ecologically harmless. Zeolite is an environmentally friendly crystalline mineral which presents an innovative alternative for adsorption, especially when utilized via Fahrenheit’s patented zeolite coating process to achieve higher capacities.
Since adsorption is a finite process, regeneration (desorption) is required to repeat the cycle. Desorption causes the adsorbed water vapor to be released via the supplied heat.
What is Zeolite?
Zeolites are a group of substances of crystalline aluminosilicates. They are environmentally friendly, harmless materials with ideal properties for adsorption. Fahrenheit developed a process called PST – Partial Support Transformation in order to optimally apply the zeolite to the heat exchanger. This process, via sophisticated chemistry, makes zeolite crystals grow directly out of the surface of the heat exchangers. This alleviates the need for any kind of coating or glue and optimizes heat conductivity. Because zeolite can reach even the smallest cavities of porous or fibrous material, the surface-to-volume ratio and therefore power density can be maximized.
Additionally, the zeolite layer forms a very effective protection against corrosion. Without adhesives or glue in between the crystal and the heat exchanger, heat conductivity is maximized.
Based on our innovations in material science and design, we can achieve a significant reduction in the size of the adsorption modules for a given cooling power. Therefore weight, size, and cost can be improved simultaneously.
In adsorption cooling technology, when extreme operating conditions are encountered, zeolite is often the adsorbent of choice.
What is silica gel?
Like zeolite, silica gel is an environmentally friendly, harmless material with properties that are very suitable for adsorption. It is commonly used in dehumidification applications and has proved itself in the adsorption field.
As an adsorbent, silica gel covers a very wide and flexible drive temperature range that is suitable for many applications.
What is the difference between an absorption chiller and an adsorption chiller?
As opposed to absorption refrigeration technology, where only liquid working materials are used or recirculated, the Fahrenheit adsorption systems are based on the principle of solid sorption (adsorption). Here, a solid working material (silica gel/zeolite) is not constantly, but cyclically in contact with the evaporating refrigerant (desorption and adsorption alternate).
By combining two modules, a quasi-continuous operation can be achieved. This is characterized by periodic slight temperature fluctuations, which can be smoothed by appropriate buffers.
The main advantage of the adsorber is that no pumping of liquids is necessary and there are no moving parts. Adsorption chillers have a simple and compact design.
In addition, low drive temperatures are a decisive difference. In contrast to absorption chillers, our adsorption chillers already perform well at temperatures starting from 50°C. As a result, the sometimes lower COP, i.e. thermal efficiency, can be compensated for.
What is the effect of the non-constant cooling capacity or the resulting cold water temperatures on the overall system? How can temperature fluctuations be minimized?
Cooling occurs quasi-continuously, resulting in periodic, short-term temperature fluctuations of up to 5K. These fluctuations can be smoothed by appropriate buffers. In general, however, these temperature fluctuations are hardly noticeable due to the inertia of the cooled thermal mass and have no practical relevance for the climate in a room.
Which refrigerant is used, and what are the restrictions?
Water is used as a refrigerant. In contrast to other refrigerants, water as a natural substance is completely non-toxic and ecologically harmless. The preset frost protection of the refrigeration system also ensures that the chiller is switched off automatically at a cold water temperature of 4°C. For the eCoo/Zeo, a temperature range from 8 to 21°C is specified for the chilled water circuit.
Which maximum cooling and heating power can be achieved by adsorption systems?
Basically, our individual adsorption chillers offer capacities ranging from 16 to 100 kW, but there is no restriction on the size of an adsorption system. The advantage of Fahrenheit’s modular approach is that customized solutions for larger cooling demands with specific conditions are also possible.
Is it possible to install Fahrenheit adsorption chillers outdoors?
In principle, an outdoor installation is possible, but not recommended. In the event of an outdoor installation, the adsorption chiller must be roofed for protection against rain, snow, and frost. Appropriate measures have to be taken in order to prevent the freezing of the hydraulic system. Freezing of the process water within the adsorption modules, however, is not critical.
Can Fahrenheit adsorption chillers also be used as heat pumps?
Yes. All chillers can be used both as a cooling system and as a heat pump.
In order to operate the eCoo/Zeo in heat pump mode, the recooling circuit or hot water circuit and the chilled water circuit have to be exchanged externally. This can be achieved via three-way switching valves, depending on the installation. In this case, either the recooler or the collector field are applied as a low-temperature source, and, for example, a cooling ceiling or a floor heating system are used for a heat emission into the room.
However, there are some limitations. The low-temperature source should not be colder than 10°C, and the maximum supply temperature is 35°C.
Is there a start-up phase during which no cold is produced, and how long does it last? Does the chiller subsequently produce chilled water at the desired temperature level?
When the adsorption chiller is switched on for the first time, an initial start-up time of up to 15 minutes has to be taken into account, since it must run a complete desorption prior to the first adsorption phase (and thus cooling). Depending on the temperature level in the chilled water cycle at the outset, a certain time is required to reach the desired chilled water temperature. This also means that power cycling the chiller is generally not energetically efficient.
eCoo/Zeo chillers are connected to three temperature circuits – the hot water (drive) circuit HT, the recooling circuit MT as well as the chilled water circuit LT. What is the effect of different temperatures in these circuits on the performance of the chiller?
The relationship between the temperatures of these three circuits and the cooling power respectively the COP is best represented by the performance charts provided.
- Higher drive temperatures increase the cooling power;
- Higher chilled water temperatures increase the cooling power and improve the COP;
- Lower recooling temperatures increase the cooling power and improve the COP.
What is the difference between eCoo and Zeo adsorption chillers?
The core of both technologies is a material which adsorbs water vapor (the adsorbent). Fahrenheit uses either silica gel, which has a very flexible drive temperature range, or zeolite crystals which, depending on their type, are particularly suitable for special conditions, e.g. hot climate zones, compact applications, or low drive temperatures.
In the eCoo adsorption chillers, silica gel is used as an adsorbent, while in the Zeo adsorption chillers, the adsorbent is zeolite.
(See also the questions about adsorption materials further up.)
In which applications can adsorption chillers be used?
Basically, the realm of potential adsorption applications is almost unlimited. An adsorption system is economically viable as long as excess heat is available.
Please find more information about common adsorption cooling applications here.
Are you planning further development?
Yes, we continuously develop and evolve our products. Improving the performance and efficiency, reducing cost, volume, and weight, as well as broadening the fields of application are among of our development goals.
Components and Materials
What are the core components of the chiller?
Fahrenheit eCoo/Zeo adsorption chillers basically consist of these components:
- One or multiple pairs of processing modules
- Silica gel or zeolite coated adsorber heat exchangers
- Switching unit incl. high-efficiency pumps
- Controller and measurement system
- Frame or supporting structure with cladding
Are the chillers prone to corrosion?
The cooling module itself is resistant to corrosion since no oxygen is contained in the system, and it consists of materials such as stainless steel and copper. The supporting structure as well as the frame are generally protected against corrosion by surface coating, sufficient for a typical interior setting. If the chiller is to be permanently exposed to higher humidity, however, a special anti-corrosion coating may be needed.
Does the silica gel or zeolite degrade?
There is no degradation or erosion whatsoever. The process of adsorption is completely reversible.
Is there any risk to the environment from the materials used?
No, there isn’t. Fahrenheit mainly uses steel, copper, and aluminum in our products. Moreover, silica gel and zeolite are naturally occurring materials which are completely harmless. The chillers can be disposed of normally after the end of the service life.
What is a HybridChiller?
The HybridChiller combines adsorption and compression cooling. It blends the environmentally friendly and energy efficient adsorption technology with the precision and power of compression.
In the HybridChiller, the adsorption part covers the main cooling load, while the compression part is engaged whenever a higher cooling load is required, or when there are fluctuations in the heat supply for the adsorption system. The combination results in maximum energy efficiency.
Are all of Fahrenheit adsorption chillers available as HybridChillers?
Yes, all our adsorption chillers can be combined with compression chillers.
Drive Source (Heat Source)
Why is a drive source/heat source needed, and what does it have to provide?
In contrast to conventional air conditioning units, Fahrenheit adsorption chillers utilize heat in form of hot water as a drive source instead of electricity. As a result, the cost-effective thermal energy can be re-used, while otherwise this energy would be released into the outside environment, or re-cooled expensively.
eCoo/Zeo save primary energy and achieve greater independence from increasing electricity prices and power grid bottlenecks. Thus, they enable environmentally friendly and resource-saving cooling.
Which drive source can be used if there is not sufficient waste heat?
From the point of view of energy consumption, the most sensible heat source for an adsorption chiller is waste heat. If the waste heat required for operation is insufficient, compression technology can be used. This is possible by using the Fahrenheit Fahrenheit-HybridChiller, or a separately installed compression chiller. Using such a combination, fluctuations in the supply of waste heat can be resolved and peak loads are efficiently covered by compression.
In theory, adsorption chillers can also be driven directly, for example with a boiler. However, this is inefficient from an energy point of view and often does not pay off from an economic point of view. Such configurations should only be used in exceptional cases.
What happens if the drive temperature HT_IN temporarily exceeds the specified maximum temperature? Can this temperature excess be prevented?
The drive temperature limit is primarily designed to accommodate the temperature tolerances of individual materials in the chiller. However, some safety margins are factored in, so that in principle a short-term overrun of the maximum temperature is harmless.
Excessively high drive temperatures must be prevented by external measures, for example by the return admixture, which is a common technique in heating technology.
What happens when the required temperatures are not achieved? Is cold production still possible?
Even at drive temperatures (HT) below 55°C, cold is still “generated”, which means the chiller does not switch off automatically. This has the benefit of compensating short-term drops in temperature in the interest of continuous cold generation and to avoid power cycling. However, this situation is not desirable for continuous operation, since the demand for electrical energy (for example, for re-cooling) still exists at lower refrigeration capacity. A limitation of the drive temperature “downwards” can be achieved, e.g., via an optional hot water buffer tank sensor.
How many square meters of collector area should be provided for the operation of the eCoo/Zeo?
As the answer depends on the varying efficiencies of the various types of collectors, as well as on the geographical location of the installation, a general statement is not possible. A rule of thumb is a 3-3.5 m² collector area per 1 kW of cooling capacity. In any case, an accurate drive power calculation is necessary after determining the desired system performance.
Why is recooling necessary?
Like all thermally driven refrigeration systems, the eCoo/Zeo also relies on recooling both the supplied drive energy and the produced/removed cooling energy. The recooling ensures that the energy adsorbed (during evaporation and desorption) can be released back into the environment. If the recooling temperature increases, the performance of the chiller generally decreases.
The thermal recooling energy can also be utilized sensibly (for example for drying processes, swimming pool heating etc.). In such setups, the overall efficiency is particularly high.
What are possible recooling solutions?
For our refrigeration systems, we offer as an optimally designed recooler with EC fans and optional fresh water spraying. This recooler adjusts its recooling capacity to the current load and thereby saves electrical energy. With the optional fresh water spraying, the amount of water is limited and only sprayed above definable outside temperatures.
In addition, wet cooling towers, geothermal probes, sea heat exchangers, wells, rivers, swimming pools or other heat sinks can be used for recooling.
Which type of recooling is recommended?
The recoolers offered by us can always be used when outside temperatures are moderate and other outside temperature-independent heat sinks are not available.
Since ground probes or sea heat exchangers cannot be applied everywhere, other recooling variants should be included depending on the application and location. In particular, the integration of a wet cooling tower can be a sensible solution at high outside temperatures.
How is the recooler controlled via the eCoo/Zeo? Is the control preset, or are there activities which still have to be performed in the initial start-up?
The rotational speed of the recooler ventilators as well as the optional spraying are controlled by the controller of the chiller according to the current operating state.
Why is the fresh water spraying limited to 400 h/year? What happens if a longer spraying time appears necessary?
In order to conserve water and to avoid any deposits on the recooler heat exchanger, the spraying of the recooler is limited to those 400 hours with the expected hottest outside temperature. This corresponds to 4 m³ per year of water. If a smaller amount of water is to be used for recooling purposes, the number of spraying hours can be reduced. Increasing the number of spraying hours beyond the 400 h/a requires appropriate water treatment to improve its quality.
Does the water spraying incur a risk of legionella?
No. Compared to wet cooling towers, where circulating water may be contaminated, in the sprayed recooler a very small amount of water is injected into the airflow for a limited time. Any moistened lamellae will dry completely during the cycle. Moreover, a special “legionella circuit” is implemented into the controller so that any water remaining in the recooler’s supply lines is removed daily.
What is an EC fan?
EC stands for electronically commutated. In comparison to the conventional AC fans, EC fans are continuously adjustable without additional equipment and have a lower power consumption, especially under partial load.
Can the recooler also be used in winter?
Winter use requires frost protection measures. If the recooler is applied as a low-temperature source for the heat pump, it should be ensured that the hydraulics are protected against frost. In this case, we recommend operating the recooler in combination with the Fahrenheit system separation with a water-ethylene glycol mixture using a glycol concentration which corresponds to the respective location (in Germany: 34%).
What are the maximum operating costs for the recooler? What is the maximum resource consumption (water, electricity) per year?
The resource consumption depends on the power load. Due to the water spraying limitation, the consumption of water is limited to 4 m³ per year. The maximum power consumption of the recooler is specified in the technical datasheet. When properly installed, the eCoo/Zeo and eRec subsystem yield an annual performance factor of 10-14: Therefore, 1 kW of electrical energy serves to generate up to 14 kW of cold.
Low-Temperature Source for Heat Pump Operation
What is a low-temperature source, and what is it required for?
The term low-temperature source refers here to a heat source with a temperature level between 8 and 21°C. This temperature level is significantly lower than the recooling temperature level. Thus, in heat pump mode, heat is withdrawn from the environment at this low temperature level and supplied to the heating circuit (now the recooling line – MT) in addition to the applied drive power.
What types of low-temperature sources are available?
Similar to electrical heat pumps, outside air, water, ground, or solar collector arrays can be used as low-temperature sources. Due to the large variety, here are some examples:
- Dry recooler in outdoor installations or in underground garages
- Sea heat exchanger, wells, swimming pool
- Ground collectors, geothermal probes
Which low-temperature source is recommended?
Ideally, the recooling can be applied as a low-temperature source in the winter. The same device which emits waste heat to the environment in summer (for example the eRec) may serve as a low-temperature source in winter. In general, those sources are suitable which have a relatively high and constant temperature during the cold seasons, as well as requiring little electrical energy (e.g., without ventilator). These include geothermal probes which ensure a constant low-temperature between 8 and 12°C.
What happens when the low temperatures are out of the recommended range?
Exceeding the preset low-temperatures has no effect, as long as the heating water temperature (MT) is 2K higher. In general, the higher the temperature level of the low-temperature source, the higher the heat output and the COP.
Low-temperatures lower than 8°C are theoretically possible, but COP and heating power will not ensure efficient operation anymore. Furthermore, the eCoo/Zeo chillers switch off at LT discharge temperatures of 4°C for frost protection.
Please note that freezing of the hydraulics has to be avoided under all circumstances. Moreover, the heat output at temperatures below 7°C cannot be guaranteed, as the temperatures would not be sufficient to maintain the process of adsorption and desorption.
What happens when the current cooling load is lower than the minimum achievable cooling capacity of the eCoo/Zeo?
If the required cooling capacity drops below the minimum partial load range of the eCoo/Zeo (usually 60% of the maximum cooling capacity), the cold carrier (brine) is gradually depleted of energy so that it cools down to the set frost protection limit (default: 4°C). The system is then switched off. If a decrease to such low temperatures is undesirable, this can be modified by raising the frost protection limit (Controller Parameter T_Freeze). This should be taken into account especially in case of cold distribution systems such as cooling ceilings, where the dew point temperature should not be underrun.
Which room sizes can be air-conditioned using these chillers?
The size of the room to be cooled is not sufficient to estimate the cooling load. The cooling load depends on the insulation standard, windows sizes, and internal thermal sources such as computers, lights, and people.
We recommend to perform an accurate calculation of the required cooling load to properly match the eCoo/Zeo with the cooling load.
Do the chillers cool with water or with air?
The eCoo/Zeo cools a constant chilled water volume flow (cold carrier) which cools the ambient air when passing through chilled water-air-heat exchangers (such as ceiling cassettes or fan coils).
Which cold distribution systems are possible?
In particular, we recommend cold distribution systems which can operate on relatively high cold water temperatures, such as:
- Floor cooling
- Wall cooling
- Chilled ceilings
- Cooling compartments
- Concrete core temperature control
- Circulating air coolers, air distribution systems
Integration of the Adsorption Cooling System
What should be considered when combining with a CHP?
The combination of an eCoo/Zeo and CHP can be a very useful setup, which also benefits from increasing the CHP’s operating hours. The available HT temperature level (drive temperature) of a CHP is generally sufficient. In contrast to solar cooling, cold production is possible around the clock. Particularly for CHPs, the maximum HT return temperature should not be higher than a certain threshold. In order to compensate for fluctuating and temporarily higher than required HT return temperatures, we recommend installing a buffer downstream from the eCoo/Zeo. With a buffer volume of just 120L, the fluctuations are reduced to a minimum.
What is the recommended distance between the chiller and the recooler, resp. the chiller and the drive source?
A certain distance between the chiller and the recooler is necessary because the recooler has to be installed outdoors while the chiller is ideally installed inside the building. Long pipes result in greater pressure losses, which in turn requires higher pump performance. Therefore, the pipe diameters must be checked for long lines and adjusted if necessary.
Regarding the distance to the heat source, heat losses increase with distance, and therefore eCoo/Zeo should be located as close as possible to the heat source. In addition, the chiller should be installed in the immediate vicinity of the cold distribution network in order to prevent unwanted warming of the chilled water.
Project Planning and Design
Can the eCoo/Zeo be operated at higher outdoor temperatures than recommended?
At higher outside temperatures, the advantages of the Zeo systems come into play. Higher ambient temperatures are easily tolerated.
When using the eCoo/eRec subsystem, the outside temperature determines the system’s economic viability, since COP drops at very high recooling temperatures (depending on LT and HT). Where exactly the application limit is depends on several parameters, such as the relative humidity. The lower the humidity, the higher the maximum outside temperature can be for the chiller operation. Higher ambient temperatures can also be compensated by higher cold water and/or drive temperatures. Generally, in the case of very high outside temperatures (>37°C) in conjunction with high air humidity, more climate independent recooling options should be selected: For example, swimming pool heating, geothermal probes, sea heat-exchangers, or similar. At high outside temperatures and low humidity, a wet cooling tower can be the most efficient solution.
We will gladly prepare for you a design that is precisely tailored to the potential location and the expected operating conditions, including a proposal for the optimal recooling solution.
Is the application of a chilled water buffer tank recommended?
A chilled water buffer tank is not necessary. Due to the slight fluctuation of the discharge temperatures of the chillers, a buffer can be used to smoothen the operation, as well as to simplify the integration of the eCoo/Zeo. It is also possible to use a buffer tank at night, for example, when the sun no longer provides the operating power for a solar-driven eCoo/Zeo.
What solar coverage rates can be achieved?
A solar collector has an efficiency of up to 0.75, and our chillers of up to 0.6. Thus, up to 45% of solar energy is used for cooling. In terms of electrical power consumption, the achievable efficiency factor is at least 14. Therefore, 1 kilowatt of electricity can generate about 14 kilowatts of cold, with the rest of the energy coming from solar thermal energy.
Operation and Maintenance
What maintenance is needed, and how much effort will it take?
Since there are no movable parts within the eCoo/Zeo, apart from the 3-way switching valves and pumps, maintenance is not strictly required. Nevertheless, we recommend an annual system check of the integrated components and materials.
The inspection is carried out either by Fahrenheit or by trained and certified Fahrenheit partners.
How much power is saved compared to conventional air-conditioning systems?
With careful planning and design, the Fahrenheit adsorption cooling systems consume up to five times less electricity than conventional air-conditioning systems.
How can the chiller be switched on and off? Do certain cycles have to be taken into account?
The eCoo/Zeo is switched on either manually via the input panel of the controller or, if provided, by activating the potential-free contact via an external control. All pumps and the recooler are automatically switched on by the eCoo/Zeo.
Once switched on, power cycling the chiller should be avoided. Every switching causes energy losses, however it has no negative effect on the eCoo/Zeo.
What are the optimal storage conditions for the chiller and recooler?
- Adsorption chiller:
- Dry, preferably at constant temperatures between +5 and 45°C; avoid freezing
- No restrictions as long as the hydraulics are drained
By what measures can the service life of the chillers and of the recooler be increased?
The system is stable within its specified operating conditions. Special measures to enhance the service life are not necessary.