1. Introduction
Worldwide, an increase of the cooling demand is expected. In the building sector, the cooling demand is the most rapidly increasing effective energy demand [
1]. Usually, the cold generation is realized by electrically driven compression chillers. Hence, the expected development in effective energy demand will lead to an increased power requirement as well. Acting on the assumptions of the World Energy Outlook [
2], the global power requirements will increase by 60% between 2016 and 2040. Approximately 15% to 20% of this elevation will be caused by the cooling demand. Consequently, solar-assisted cooling systems (SAC systems) have a large potential to reduce the fossil fuel consumption related to non-regenerative power generation.
On the other hand, it has been shown (e.g., in [
3]) that a minimum solar fraction is necessary for SAC systems in order to achieve a lower primary energy consumption than a conventional system using an electrically driven compression chiller. Henning et al. [
3] illustrated that the system performance with regard to primary energy savings is improved when the COP of the thermally driven chiller increases, the solar fraction increases, and the specific electricity consumption of the auxiliary components like supply pumps and cooling tower etc. decreases.
For SAC systems with parallel backup heating, the achievable solar fraction is a question of the possible operating time with solar heat from the collector. Solar heat operation (SHO) can start earliest when the collector temperature is higher than a minimum driving temperature
and has to end latest when the collector temperature drops below. Of course, additional temperature differences have to be taken into account (e.g., for a heat exchanger between the collector and storage, heat losses etc.). The minimum driving temperature
depends on the cooling load as well as the capacity and part load behaviour of the chiller. Moreover, it depends on the possible cooling water temperature, which is limited by the dry or wet bulb temperature. Generally speaking,
depends on the load and meteorological conditions as well as the technology used for the chiller and the reject heat device but not on the collector technology. Nevertheless, the collector technology (including storages and the connection to parallel or serial backup heating systems) has a large influence on the necessary preheating time (i.e., the period from sunrise until
is reached and solar heat operation can start) and thereby influences the overall system performance [
4].
Taking the heat capacity effects of the collector and installation, as well as heat losses to the surrounding into account, Izquierdo [
5] determined a solar radiation threshold
as an equivalent measure to the minimum driving temperature. For a small SAC system in Madrid, two different heat rejection technologies were compared for the same meteorological conditions. In comparison to a wet cooling tower, a dry cooler revealed approximately 12 K higher cooling water temperatures. This led to about 20 K higher driving temperatures in the collector circuit and reduced the possible operating time of the SAC system from 9 to 3 h. The preheating time increased from approximately 3 to 6 h.
The influence of heat capacity effects on the preheating time was investigated by Li [
6] and Kohlenbach [
7] as well. While Li used a separated smaller part of a partitioned hot water storage tank to reduce the preheating time, Kohlenbach applied different temperature nodes of the storage tank for the control of the chiller before, during, and after solar heat operation. In both references, a constant (i.e., load and weather independent) start-up temperature of
= 75 and 60 °C was used.
In order to shorten the preheating time, which is necessary to achieve a desired solar collector outlet temperature and thereby enable a longer solar operation of a single-effect
absorption chiller, Shirazi et al. [
8] recommend a variable speed pump for the solar collector loop. A comparison of the simulation results to a constant flow strategy showed that the solar fraction can be increased by about 11% by the temperature control strategy. Shirazi et al. [
8] concluded that the longer solar operating time of the chiller (which results from the higher collector temperature) is more important for achieving a high solar fraction than the reduced collector efficiency due to a higher collector temperature.
Comparably, Qu et al. [
9] investigated a constant flow rate versus constant outlet temperature control by transient simulations of an SAC system with parabolic trough collectors and a double-effect
absorption chiller. They also indicated that the constant temperature control reduces the preheating time in the morning by approximately one hour and extends the operating time of the chiller driven by solar energy by approximately half an hour in the afternoon.
To make low collector temperatures applicable, Clauß et al. [
10] described a control strategy similar to the one mentioned in [
11] but without the constraint of an overflowing evaporator. Clauß et al. [
10] used a control strategy based on the characteristic equation method that determines the required cooling water temperature for a predetermined driving temperature (e.g., from the solar collector field) in order to match a certain cooling load and to maintain the chilled water set value. In addition, using the cooling water temperature as the manipulated variable offers the possibility to save electric power and also water—if a wet cooling tower is used—provided the available driving temperature from the collector is higher than necessary to cover the load. In this case, a higher cooling water temperature can be used, which reduces the electricity demand of the reject heat device.
Unfortunately, the characteristic equation applied by Clauß et al. [
10] is valid only for the 10-kW absorption chiller under investigation (i.e., type sun
inverse). Furthermore, the external supply flow rates in hot, cooling, and chilled water circuit must agree with the nominal flow rates, because otherwise the slope and loss parameter in the characteristic equation would change. Moreover, Clauß et al. [
10] did not report on the reverse applicability of the characteristic equation method, i.e., calculating a necessary driving temperature from a given cooling water temperature in order to keep the preheating time short.
In this contribution, a model predictive control algorithm is developed, which allows an increase of the solar operating hours of an SAC system by calculating the minimum possible driving temperature under the constraint of a maximum allowed auxiliary electricity demand to ensure a short preheating time. This new strategy utilizes an improved and more precise method of characteristic equations [
12]. The computer code of the method is available in [
13]. The same characteristic equation method is used to control the absorption chiller itself, irrespective of whether it is operated in a solar or conventional cooling system [
14]. Since the chiller control is based on characteristic equations, it is called a CE controller. Nevertheless, the focus of this contribution is not on the absorption chiller control (which is done by the CE controller and is described, e.g., in [
14,
15]), but on the control of the switchover between solar and backup heat operation of the chiller in a solar cooling system. Hence, following a short description of the SAC system at the Federal Environment Agency in Dessau, Germany (which was used to test the switchover strategy), the improved characteristic equation method is explained only briefly. Afterwards, the model predictive control algorithm for the switchover strategy is derived. It combines the absorption chiller model with a simple dry-cooler model. Finally, the measured results of the strategy are shown and discussed.
3. Results—Model Predictive Control
The model predictive control strategy for an extended solar heat operation of the SAC system combines the absorption chiller model (i.e., the improved characteristic equation method) with a part load model for the reject heat device. Since the combination of absorption chillers with dry coolers has become increasingly more common and the humidifier at the Federal Environment Agency in Dessau is normally blocked, a dry-cooler model is sufficient. After the description of the dry-cooler model, the control strategy is derived.
3.1. Dry-Cooler Model
In
Figure 5, the nomenclature for the dry-cooler model is depicted. A control signal
is used to adjust the air flow rate
by changing the rotational speed of the ventilators or their motors, respectively. The necessary electrical power of the dry-cooler
for a certain air flow rate
is proportional to the pressure drop and total efficiency
, including all mechanical and electrical losses of the motor-ventilator assembly. In addition, a constant auxiliary power consumption
can be considered (e.g., for a control unit). Assuming a quadratic development of the air-side pressure drop
with respect to the pressure drop under nominal conditions (i.e.,
), this leads to:
which can be rewritten by expanding the fraction with
as:
where
is the normalised flow rate or the control signal of the dry cooler, respectively, and
is the electrical power under nominal conditions.
The unknown air flow rate
, which causes the electrical power,
for a certain load condition, results from the reject heat flow of the absorption chiller from the absorber and condenser,
. It is equivalent to the negative sum of the evaporator and desorber heat flow:
To determine
(and thereby
), the required air outlet temperature
in Equation (13) can be calculated from the dimensionless temperature glide,
of the air stream, assuming a counter flow heat exchanger (i.e., neglecting all cross-counter flow effects):
The required cooling water inlet temperature of the dry-cooler
on the left-hand side of Equation (14) is equal to the condenser outlet temperature,
of the absorption chiller. It follows from the inlet temperature
plus the temperature increase in the cooling water flow of the absorption chiller due to the total reject heat flow
. Since the cooling water valve is used only for safety reasons, it is normally fully open. Consequently, for serial cooling water flow, the flow rates in the absorber and condenser and in the dry-cooler are the same, i.e.,
:
Combining Equations (14) and (15) results in:
and subsequently inserting Equation (16) into Equation (13) with the ambient air temperature as the air inlet temperature
leads to:
However, Equation (17) determines
only implicitly, because
is a function of
since the dimensionless heat transfer capability
and heat capacity flow rate ratio
in Equation (14) depend on
:
In order to solve Equation (17) explicitly for
, an approximation
for the dimensionless temperature glide,
is used from [
12], appendix O:
In this equation, holds for the air side capacity flow rate at the nominal condition (Index 0) and is calculated with (i.e., also at the nominal condition). Thus, at the normalized flow velocity , the approximation equals (i.e., the exact value). For , Equation (19) converges to the same limit value as in Equation (14). Between and , the exponential dependency is considered by a reciprocal approach.
Inserting approximation Equation (19) into Equation (17) leads to an approximated but explicit equation for the air volume flow rate
or the normalized flow velocity
when constant property data can be assumed:
With
determined, e.g., from manufacturers’ data under nominal conditions or from measurements at a single reference point, and the coefficients:
the approximated electrical power
under the part load condition can be calculated:
A comparison between the measured and approximated values for the electrical power (i.e.,
and
) is depicted in
Figure 4a for the operating period discussed in
Section 2.2. without the humidification of ambient air.
3.2. Switchover Control Strategy
The aim of the following control strategy is to find a cut-in and a cut-off condition for the solar heat operation (SHO) of an absorption chiller cooling assembly (ACCA) under the constraint that the specific electricity demand during SHO is lower than the electricity demand of a reference cooling technology (e.g., a compression chiller cooling assembly, CCCA).
The specific electricity demand of the ACCA is dominated by the electricity demand of the dry cooler. Thus, a simplified cut-in condition for SHO is to allow a maximum control signal
, which results in an approximated specific electricity demand
lower or equal than the specific electricity demand of the compression chiller cooling assembly, i.e.:
Solving for the maximum allowed
leads to the boundary value:
This depends on the reference cooling technology (
), load condition (
), and electrical values of the reject heat technology of the SAC system (
,
). In addition, the control signal of the air flow rate
for the same cooling load
and the necessary driving heat flow according to the characteristic Equation (8) is determined by Equation (20), hence:
In this equation, only the supply temperatures and are unknown, because is fixed by the load and technology parameters according to Equation (25). On the right-hand side, in and are fixed by the load and weather condition and the coefficients , , and describe the thermal part load behaviour of the dry-cooler and absorption chiller.
The cooling water inlet temperature
in Equation (26) can be eliminated by the rearranged characteristic Equation (7) in combination with the characteristic temperature in Equation (6), i.e.:
After inserting Equation (27) into Equation (26), it can be solved for the driving temperature
, which is necessary for a cooling load
at a chilled water temperature of
and a maximum allowed control signal for the dry cooler
in order to keep the specific electricity demand
below
when the absorption chiller is operated with solar heat:
where:
Finally, the operating limits of the absorption chiller have to be considered. For example, to avoid crystallization of the
solution, a minimum cooling water temperature
is allowed. Hence, from the rearranged characteristic Equation (7) follows a minimum driving temperature for the load case
:
Consequently, the necessary driving temperature to start the solar heat operation is the maximum of the aforementioned temperatures:
The cut-in condition for solar heat operation (i.e.,
) becomes true when the highest storage temperature
and the outlet temperature of the solar heat exchanger
are both higher than the necessary cut-in temperature for solar operation
:
The cut-off condition for SHO consists of three aspects: Solar heat operation should be stopped when the highest temperature in the solar heat storage
has decreased below the necessary driving temperature
to cover the load or the necessary driving temperature
for SHO, and simultaneously, the electricity demand becomes too high or the load cannot be matched anymore. Thus, the cut-off condition reads:
Typical values for the thresholds and are 2 and 0.2 K, respectively.
4. Discussion
The model predictive switchover strategy for extended solar heat operation (SHO) with improved cut-in and cut-off conditions has been implemented into the industrial programmable logic controller (PLC) of the single-stage
-absorption chiller of type FM050v0.3 operated at the Federal Environment Agency (UBA) since 2011. Nowadays, this chiller is marketed as type “Bee” [
19]. Via a Profi-Bus-connection, the information for the control signals of the flaps
and
is sent to the building management system, where it is put into execution. The switchover from BHO to SHO without a model-based cut-in condition is described in
Figure 2. Now, the improved switchover strategy including a model-based cut-in/off condition is described. The measurements and overall results of the strategy are depicted in
Figure 6 for an operating period of 2 days in September 2019. In
Figure 7 and
Figure 8, the cut-in and cut-off time periods are shown separately. On the first day, solar heat operation of the absorption chiller started at 11:00 a.m. and ended on the next day at approximately 08:00 a.m. Thus, the duration of the solar heat operation period is 21 h in comparison to 13 h of sunshine duration, i.e., from 06:30 a.m. until 07:30 p.m.
The driving temperature
used in SHO started with approximately 67 °C, increased up to 78 °C in the afternoon of the first day, and decreased down to 49 °C in the morning of the next day. The corresponding cooling water temperatures
of 25, 29, and 19 °C, which are necessary to cover the nearly constant cooling load of 23–29 kW
0, were supplied by the heat rejection device in dry-cooler mode with a control signal
of less the 60%. Accordingly, the mean specific electricity demand during the whole SHO period
0.18 kW
el/kW
0 is approximately 25% below the average specific electricity demand of the reference cooling technology
0.24 kW
el/kW
0 (cf.
Figure 6d).
Inserting the electrical values
= 13.5 kW
el and
= 1.7 kW
el of the reject heat device at UBA into Equation (25) together with
= 0.24 kW
el/kW
0 and the measured cooling load of about 23 to 29 kW
0 leads to a maximum allowed control signal for the dry cooler
of 0.66 to 0.73. Instead of this variable value, a constant value
= 0.6 was used for test purposes. The resulting necessary driving temperature to start the solar heat operation
of about 60 to 64 °C during the preheating time is plotted in
Figure 7. At 10:50 a.m., the cut-in condition became true (cf. Equation (34) and
in
Figure 7) and a delay timer of 10 min was started. Since
was true over the full delay period, the information for the control signal
of the flap to be opened and
of the flaps to be closed was sent to the building management system via Profi-Bus. Thus, at 11:00 a.m., solar operation started with a lower desorber inlet temperature
67 °C as before (i.e.,
74 °C during BHO with district heating, cf.
Figure 7). If the cut-in condition became false (i.e.,
) during the delay period, the timer would have been reset to zero and restarted when
had become true again. Thereby, a cut-in delay was realized.
Due to the lower driving temperature
67 °C (which is a mixed temperature of
70 °C from the solar heat exchanger and
65 °C from the storage), the set value for the cooling water
was decreased by the CE controller for a while in order to match the cooling load and chilled water set value,
. Therefore, the controller of the dry-cooler increased the control signal
to adjust
to
(cf.
Figure 6a). Shortly after 11 a.m.,
also increased above the cut-off value
= 0.5 and the cut-off condition
according to Equation (35) became true. Thus, for a definite result,
and
have to be used as set and reset inputs in a bistable function block.
With the rising ambient air temperature until 04:00 p.m., the necessary driving temperature
also increases according to Equation (28). Since the available temperature from the solar collector and storage is high enough, SHO is ongoing. At nighttime, the available storage temperature
falls below
Nevertheless, the second part of the cut-off condition in Equation (35) is not true. Due to the low ambient air temperature, a low cooling water temperature is also available to counterbalance the low driving temperature. Thereby, the chilled water set value is still matched without an increase of the specific electricity demand
(cf.
Figure 6d). From approximately 06:30 a.m. of the second day on, the cooling water temperature
is limited by the boundary value
= 19 °C (cf.
Figure 8). With a continuously decreasing driving temperature
below 50 °C, the evaporator outlet temperature
starts to increase and the cooling load is not matched anymore. Consequently, SHO has to be stopped approximately at 07:30 a.m.
The rapid increase of
at approximately 7:45 a.m. is an effect of a switching action in one of the heating circuits, which causes a much lower hot water flow rate
for some minutes. For flow rates close to zero, the slope coefficients
and
(i.e., the possible capacity of the chiller or its ‘thermal size’) become very small and consequently the necessary driving temperature
high. Unfortunately, this interference happened in parallel or at the end of the cut-off delay for SHO. Nevertheless, it did not switch off the solar heat operation (although it looks like it) and did not have any effect on the end of SHO at all. The cut-off condition (cf. Equation (35) or
in
Figure 8) was true since 07:10 a.m. With the cut-off delay time of 20 min, SHO was stopped at 07:30 a.m. A very few minutes later (i.e., after communication between PLC and the building management system), the discharge pump P35 was switched off and the flow rate
reached zero. Now, SHO also stopped hydraulically. Incidentally, at the same moment,
increased after the short interruption.
The potential of the improved control strategy was illustrated by a theoretical application during the test period in 2017 (cf.
Figure 2), when the cooling load was low during the preheating time in the morning of the first day (cf.
Figure 3). Under these operating conditions, the cut-in condition
would have become true before 10:00 a.m. and a necessary solar driving temperature below 65 °C at the outlet of the solar heat exchanger
would have been sufficient to start SHO.
5. Conclusions
For solar cooling systems with an absorption chiller cooling assembly (i.e., including all the supply pumps and the heat rejection device), the possible operating time with solar heat is a key parameter to achieve high solar fractions and thereby high primary energy savings. Hence, for SAC systems, the solar heat operation (SHO) of the absorption chiller should be started as early as possible and should be continued as long as possible in order to reduce the backup heating demand, thereby reducing the main source of the primary energy demand. On the other hand, when the SHO mode is activated too early and/or stopped too late, the available driving temperature from solar collector is low. Consequently, a lower cooling water temperature is necessary to cover the cooling load. It depends on the part load behavior of the chiller and can be calculated by the characteristic equation method. The lower cooling water temperature causes a higher electricity demand in the reject heat device. These theoretical interdependencies were exemplified during a test period with continuous solar heat operation over more than three days at the SAC system of the Federal Environment Agency in Dessau, Germany. It was shown that the resulting electricity demand during solar heat operation might exceed the demand of the reference technology (e.g., a compression chiller cooling assembly) and no primary energy savings are possible anymore.
In order to extend the solar heat operation period on the one hand, and to ensure a specific electricity demand below a boundary value on the other hand, a model predictive switchover strategy was developed. The combination of an improved characteristic equation method for the part load behavior of absorption chillers with a simple dry-cooler model was used to find a suitable cut-in and cut-off condition for the solar heat operation. The two conditions depend on a minimum solar driving temperature, which has to be reached by the solar collector and storage before solar heat operation is enabled. The minimum driving temperature depends on technical coefficients describing the part load characteristics of the dry cooler and absorption chiller, and also on the load and weather conditions.
Measurements at a solar cooling system for an IT center with a cooling demand of approximately 20–30 kW0 around the clock showed that solar heat operation could be started with approximately 67 °C and was possible down to 49 °C. The duration of solar heat operation was 21 h in comparison to 13 h of sunshine duration, and the mean specific electricity demand during the whole SHO period was approximately 25% below the average specific electricity demand of the reference cooling technology.
Although the model predictive switchover strategy utilizes the same method of characteristic equations as the control algorithm of the relevant absorption chiller in the investigated SAC system, it is generally possible to apply the switchover strategy independent of the chiller control. In contrast to the state of the art, where constant (or at least load and weather independent) cut-in temperatures for solar heat operation are used, the new switchover strategy incorporates the actual load and weather condition as well as the part load capability of the absorption chiller and reject heat device. Thus, the new switchover strategy provides the cut-in temperature as a quality of the whole absorption chiller cooling assembly supplied by solar energy.