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CHEMICAL ENGINEERING TRANSACTIONS
VOL. 43, 2015
A publication of
The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet
Chief Editors:SauroPierucci, JiříJ. Klemeš
Copyright © 2015, AIDIC ServiziS.r.l.,
ISBN 978-88-95608-34-1; ISSN 2283-9216
An Experimental Approach for the Dynamic Investigation on
Solar Assisted Direct Expansion Heat Pumps
Federico Scarpaa, Andrea P. Reverberib, Luca A. Tagliaficoa, Bruno Fabiano*,c
aUniversity of Genoa, DIME/TEC Thermal Energy and Environmental Conditioning Division, Via Opera Pia 15a, Genoa, Italy
bUniversity of Genoa, DCCI Department of Chemistry and Industrial Chemistry, via Dodecaneso 31, Genoa, Italy
cUniversity of Genoa, DICCA Department of Civil, Chemical and Environmental Engineering, Via Opera Pia 15, Genoa, Italy
bruno.fabiano@unige.it
In this paper, a direct expansion solar assisted heat pump (DX-SAHP) is investigated as one of the possible
low environmental impact solutions to the domestic thermal energy demand, even if due to some technological
and control strategy limitations, the DX-SAHP systems do not have a competitive role in the actual renewable
energy market. An instrumented prototype of a small DX-SAHP system is here designed and presented, within
the context of developing safe and affordable sustainable energy applications, to determine the best control
strategies and to measure the energy performance indices with good reliability and accuracy. The property
calculation tool and the main issues related to the performance measurements and calculations are reported,
with an estimation of the measurement error. The results of test series evidence that the apparatus can fulfil
significant thermal loads, with a coefficient of performance (COP) up to 6.
1. Introduction
Applications of clean energy power systems are evolving in a wide range of industrial sectors and different
geographical locations while, considering the depletion of fossil fuels, a broad debate has long been opened
within both the scientific and political communities concerning the strategies to be adopted for sustaining the
increasing world energy demand (Nemet et al., 2012) and side-on effects of potential solutions (Milazzo et al.,
2013). Additionally, the whole problem of energy supply and energy demand reduction has a wide
interdisciplinary character owing to its social, environmental (Fabiano et al., 2012) and safety implications
(Palazzi et al., 2014) in both industrial and domestic-scale systems. Solar energy based systems may play a
positive role in integrating and phasing out fossil fuels, so reducing the risk connected to different accident
scenarios (Palazzi et al., 2013) and consequently process accidents and near-misses (Fabiano and Currò,
2012). Solar energy is attracting more and more scientific interest in several processes, e.g. water splitting by
photo-electrochemical reactors (Hankin et al., 2014). Within this broad context, although the solar assisted
heat pump concept has been known since more than fifty years (Sporn and Ambrose, 1956), only in the 90’s
advanced technological characteristics like variable capacity compressor (VCC) were available for small
refrigeration plants (Chaturvedi et al.,1990). Additionally, more sophisticated prediction and control techniques
were recently developed, including moving phase interface modelling (Dong et al., 1998) and proper state-
space representation (Schurt et al., 2009), making it possible to apply this inventive solution effectively. Due to
these technological and control strategy limitations, the Direct Expansion Solar Assisted Heat Pump (DX-
SAHP), also known as Integrated Solar Assisted Heat Pump (ISAHP) technology, does not have a competitive
role in the actual renewable energy market, despite several solar-assisted heat pump configurations were
investigated since the 80’s, as from the comprehensive review by Ozgener and Hepbalsi (2007). Several DX-
SAHP devices were presented in the scientific literature as swimming pool heating source (Tagliafico et
al.,2012a), commercial small refrigeration appliances (Omojaro and Breitkopf, 2013), or as the evaporator, by
using a roof-integrated solar collector (Yang et al., 2011). The results of previous studies usually suggest that
proper design and optimized control criteria are recommended to exploit at best this renewable energy
technology (Scarpa et al., 2011). Easy-to-handle thermodynamic models relying upon both a steady-state
DOI: 10.3303/CET1543415
Please cite this article as: Scarpa F., Reverberi A., Tagliafico L.A., Fabiano B., 2015, An experimental approach for the dynamic investigation
on solar assisted direct expansion heat pumps, Chemical Engineering Transactions, 43, 2485-2490 DOI: 10.3303/CET1543415
DOI: 10.3303/CET1543415
Please cite this article as: Scarpa F., Reverberi A., Tagliafico L.A., Fabiano B., 2015, An experimental approach for the dynamic investigation
on solar assisted direct expansion heat pumps, Chemical Engineering Transactions, 43, 2485-2490 DOI: 10.3303/CET1543415
2485
(Scarpa et al., 2013a) and a dynamic (Scarpa et al., 2013b) analysis of a DX-SAHP system can assume a
basic role in multi-variable multi-control refrigeration systems, where at least the compressor capacity
(variable speed compressor) and the expansion valve characteristics (electronic expansion valve) can be
controlled independently from each other. This technology, only recently introduced in the market, is not yet
duly exploited in current refrigeration and heat pump systems, so that a reliable experimental modelling
method is required in investigating the most suitable configuration under different technical and environmental
conditions. This paper aims at presenting an experimental approach as an innovative strategy to sustainable
and safe solar energy applications, consisting of an instrumented patented DX-SAHP system (De Filippis et
al., 2010) (DX-SHAP machine, water loop, acquisition system, properties calculation program) allowing to
reliably measure the working parameters of the heat pump and to start the preliminary work needed to design
an “embedded” control system using modern IC technologies. The experimental set up will be used to analyse
the best control strategy able to maximize the energy savings and system efficiency, for given user thermal
energy demand and environmental working conditions. The designed set up was realized in order to analyse
the real capabilities of this technology and with the purpose of validating the simulation results. Preliminary
results are then presented, showing how the DX-SAHP system can be well adapted to produce medium
temperature (around 323 K) hot water in winter season (i.e. at very low external ambient temperatures),
keeping good efficiency of the heat pump system (heat pump COP within the range 4.5 -6.5), with collector
efficiency almost double than traditional solar flat plate panels and connected overall plant intensification.
2. Experimental set-up
The DX-SAHP machine consists of a traditional inverse (refrigeration) cycle (Scarpa et al., 2011) equipped
with a Variable Capacity Compressor (VCC) and an Electronic Expansion valve (EEV). A consistent modelling
approach for a thermosyphon heat-exchanger was proposed by Chyng et al., (2003). The project is primarily
designed to realize a high-performance plant solution, using existing, commercial components, so that the
cost and number of special components have been kept as low as possible. The experimental apparatus (see
Figure 1) comprises the DX-SAHP machine linked to a thermostatic bath operating as a virtual user
boiler/heater, regulating the temperature of the water loop which is coupled to the condenser of the
refrigeration system. The thermostatic bath controls the temperature with an accuracy of 0.1 K and is
equipped with a 25 L tank and a circulation pump. The solar collector, where the refrigerant boils, is realized
with a bare Al flat plate of about 1 m2 (0.97 x 1.03 m) surface. A copper coil is wounded under the panel,
which is the refrigerated plate. The surface of the panel has been treated with a high absorptivity black
coating. The compressor, which is a 6 cm3 VCC (variable speed device) HBP (High Back Pressure) hermetic
apparatus, is adapted for working in a higher temperature range with respect to its design working parameters.
The condenser is a coaxial pipe heat exchanger, with water circulating in the outer section loop, operating with
an additional flow path through a specific sub-cooler water tank. The whole structure is well insulated with
expanded polypropylene. To easily match the user demand with the system (solar) working conditions, a small
20 L water tank was used as a sub-cooler section, before the refrigerant reaches the expansion valve. In this
way the EEV is fed with a well-established liquid flow rate, assuring a good repetitiveness of the control
operation. The valve installed in the prototype is a Pulse Modulated Electronic Expansion Valve. This type of
valve is easily adaptable to different temperature and pressure working operation ranges, depending on the
heat transfer rates needed and the temperature span between the evaporator and the condensing unit. The
valve has a duty cycle of 6 s. The external water loop is realized with insulated rubber pipes (1/2” diameter).
The inlet temperature (in the DX-SAHP) of the water is fixed by the thermostatic bath. A system of artificial
lighting controlled via computer produces a time-varying radiation on the prototype’s panel, with a colour
temperature of ca. 2300 K and a maximum mean irradiation on the panel surface of 800 W m-2. Even if the
radiation spectrum is moved towards the infrared region with respect to solar radiation, the system is suitable
for the study purposes, allowing great flexibility in the experimentation procedure, independently of weather or
sun shine conditions. A phase angle controller modulates the power supplied to 6 halogen 400 W lights,
allowing the system to reproduce any given time-depending radiation law. The lamps have been installed in
two racks at 20 cm from the panel surface. A radiation power map has been realized and the artificial solar
system has been calibrated using an insulated camera, a solar meter and a steel plate equipped with
thermocouples. The accurate calibration has been done for different light powers and distance from the plate.
The results of the calibration were used to correlate the actual mean irradiance on the plate to the lighting
power of the lamps and to a spot-measure performed by means of a small solar meter, 5x7 cm in surface,
placed just over the panel. A view of the experimental set-up and a layout of the plant configuration are
reproduced in Figure 1. In order to measure and collect all data required for calculating the properties and the
performance parameters of the DX-SAHP, an acquisition system has been realized together with an ad-hoc
designed data acquisition board.
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Figure 1: The DX-SAHP experimental set-up developed at DIME /University of Genoa. (a) Final configuration
with the lighting system. (b) Conceptual scheme of the set-up.
As already mentioned, the second task of the project is to develop the apparatus with an embedded control
system, including also the refrigerant thermodynamic states calculation and an automatic set-point
optimization. For this reason, some limitations in the software development have been taken into account,
trying to avoid external software references which could be inserted with difficulty in a small embedded
electronic device. The system was instrumented with 14 NTC sensors for the temperature measurements (6
on the refrigerant circuit, 2 on the panel, 1 on the compressor shell, 3 for the water loop, 2 for the ambient air
condition), 2 analogical and 2 piezoelectric manometers, 1 flow meter for the water loop, 1 solar meter and 1
wattmeter for the measurement of the electrical consumption of the compressor. Ambient air humidity has
been measured by means of dry-bulb and wet-bulb temperature measurements. The refrigerant flow rate, for
assigned compressor characteristics, has been calculated using the global energy balance described by Eqs
(1), (2), (3) and (4). Figure 1 (b) depicts the structure of the experimental set-up, including the locations of the
relevant sensors. A dedicated acquisition board collects all the raw signals and converts them in 0-5 V
electrical signals, which can be easily translated in numerical values of the physical measurements by means
of a proper calibration. All the signals collected by means of the electronic board have been sent, by means of
a standard USB data acquisition board, to a linked “target” PC. The acquisition tool, the properties calculation
tool and the control system have been gathered in one Matlab-Simulink program sheet. The Simulink
acquisition tool converts the 0-5 V signal into the data through the calibration curves and the measurements
are sent both to the control system and to the properties calculation sub-program. All sensors have been
calibrated and the measurement errors were estimated by means of the standard deviation σ values.
3. System modelling
The signals are analysed using a program developed in Matlab-Simulink. The program, based on
thermodynamic cycle calculations and a lumped dynamic system simulation, determines the thermodynamic
working condition of the DX-SAHP and it is able to send a proper set-point control signal to the variable
compressor speed and the electronic expansion valve modulation. In the meantime, all heat and power rates,
including performance indexes of the system, are evaluated and stored. In order to calculate all properties for
the refrigerant used as thermodynamic fluid, such as enthalpy (point 1,2,3,4, circled numbers in Figure1(b)),
entropy (point 1,2) and density (point 1), an interpolation has been done on a matrix of values (300) calculated
with the well-known NIST library and they have been inserted into a lookup table block. With the purpose of
converting the Simulink program by a proper compiler and of subsequently transferring it into an embedded
processor, the lookup tables were used instead of the common state function equations for the selected fluid
to avoid assigned operations (like square roots or logarithms). The sub-program included in the
aforementioned software has been designed in order to minimize the approximation error for every working
condition. The maximum approximation error occurring when enthalpy is evaluated by means the lookup table
(in particular near the vapour saturation curve) is smaller than 0.8%. The pressure calculation is carefully
verified by comparing its value with the data acquired by a pressure transducer at point 4. The usual
performance parameter COP and the heat and work transfer rates are evaluated by Eqs (1), (2), (3) and (4),
(where subscripts reference is as follows: 1 = compressor inlet; 2 = compressor outlet; 2’ = isentropic
compressor outlet; 3 = condenser outlet; 4 = evaporator inlet): = (ℎ − ℎ ) (1) = (ℎ − ℎ ) (2)
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= (ℎ − ℎ ) (3) = (4)
Isentropic and volumetric efficiencies of the variable speed drive compressor have been calculated using the
performances map data available by the compressor manufacturer. The assumed functional dependences of
these parameters on rotational speed and outlet to inlet pressure ratio follow as (Tagliafico et al., 2012b): = (5) = (ℎ − ℎ )/(ℎ′ − ℎ ) (6)
η = 0.932 − 0.087 − 0.0162 (7)
= 0.65 − 0.087 − 0.00285 − 0.000415 (8)
An important issue of this study relies upon the estimation of the measurement errors in the acquisition system
(Table 1) that may affect the properties calculation. We stress that the reliability of the measurements depends
not only on the sensors accuracy, but also on their proper location in the circuit. It is noteworthy noting that the
temperature and fluid flow rate measurements at the compressor outlet require a careful attention.
Table 1: Error prediction within the measurements range for the given data acquisition system.
Measurement Symbol Instrument Max Error
Temperature T NTC 0.4 K
Pressure p Digital pressure transducer 2.10 ⋅103 Pa
Radiation G Solar meter 18 W
Humidity RH Wet bulb hygrometer 0.3 %
Power consumption P Watt meter 1.2 W
Water flow rate Qw Flow meter 0.3 L h-1
The first problem is caused by the fact that the temperature sensors are placed upon the refrigerant pipe
(insulated) and not inside the refrigerant stream. As a consequence, a correct estimation of the inner fluid
temperature would require an accurate determination of the overall wall-fluid heat transfer coefficient
(Reverberi et al., 2013). Considering the thermodynamic system, the conductivity of the copper tube cannot be
neglected. Furthermore, the temperature of the compressor shall be significantly different from the refrigerant
temperature at point 2 (normally set between the inlet and outlet temperature). The errors affecting the
temperature measurements at point 2, calculated by numerical simulations, were significant and they required
a further ad-hoc estimation procedure. In order to estimate the maximum error for h2, its value has been
compared with the results obtained by Eqs (5) and (6). The same analysis was performed comparing the
experimental value, qcond, with the calculated value based on the energy balance according to Eqs (10) and
(3). In addition to the aforementioned error sources, it is worth remembering that the calculation of the heat
transfer rates is equally affected by the error in the mass flow rate measurement, namely by the approximation
introduced by the volumetric efficiency, according to Eq (7).
= (ℎ − ℎ ) (9) − = (ℎ − ℎ ) = ( − ) (10)
We conjectured that the use of thermocouples inserted inside the refrigerant flow and a flow meter installed
inside the refrigerant loop might be beneficial in order to reduce the errors listed in the above mentioned table.
This technical solution has been tested, but the device was blocked by oil residuals coming from the
compressor. Table 2 summarizes the results of error prediction on the estimation of the main properties of the
DX-SAHP system, calculated according to the Superposition of Errors Theorem (Leaver and Thomas, 1975).
Table 2: Global error prediction on the overall heat balance (within the application measurements range).
Measurement Max error q 18.7 % q 3.6 % P 14.1 %
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4. Results and discussion
Preliminary pilot tests were carried out to analyse the DX-SAHP capabilities in terms of performances and
working conditions limits and some reliability results are shown in Figure 2. Entering into details, Figure 2(a)
shows the heat flux for winter standard environmental climate in Genoa (Italy). Under these conditions, the
DX-SAHP machine can be integrated by a water heater keeping down the condenser temperature working
condition of the machine. Preliminary results show that when a proper refrigeration capacity and expansion
valve modulation are settled, COP values up to 6 can be achieved steadily. Available data prove that the
values are reliable, despite being affected by the estimated errors, and all the derived thermodynamic and
energy balance calculations are consistent with the physical constraints. Also due to the different winter
environmental conditions, the attained coefficient of performance values are slightly higher than the ones
reported by Yang et al, (2011) showing average COP of the heat pump system equal to 2.97 and a maximum
around 4.16. Checks of energy balance equations on refrigeration side compared to water loop side give
results in agreement within a ± 20% tolerance (Figure 2(b)). Owing to the reliable measurement apparatus
here developed, it will be possible to verify different control strategies applied on the DX-SAHP machine
during dynamic operation. Right now, it can be affirmed that the input power needs to be modulated as a
function of the external temperature and, particularly, as a function of the incident solar radiation. As amply
known, when considering a standard heat pump system, device performance can be quantified by the COP;
the higher the COP value the lower the electricity used for the same user thermal demand. Conversely, when
it comes to DX-SAHP, COP and solar collector efficiency (both depending on panel temperature) must be
taken into account. Optimal behaviour can only be achieved through a balance between two conflicting
requirements: on the one hand, a low panel temperature, next to that of the environment, to reduce the heat
losses and then use a greater fraction of the radiation available solar; on the other hand, a high panel
temperature, next to that of the condenser, to achieve high COP values, thus reducing the need for electric
power. Following the pilot test series, a further step, will be devoted to investigate system performances under
a wider range of operating conditions in order to develop an optimal and reliable control strategy.
5. Conclusions
An experimental apparatus of an Integrated Solar assisted heat Pump has been presented, together with the
data acquisition and control system implemented. The main issues related to the refrigerant fluid flow
estimation ad the measurements errors have been described and their influence on the performance indexes
calculation has been analysed. Some preliminary tests have been carried out showing the reliability of the
experimental set up, even if a ±20 % uncertainty is still present in the global COP calculations and energy
balance between the heated water-side and the heat pump fluid-side calculations. The apparatus aims at
characterizing this plant configuration, in order to identify the best control strategy. This work has a twofold
purpose: the first one is focused on testing different system control strategies in order to select the optimum
one. The second one refers to the identification of the energy performances parameters of the machine to
accurately set up the control and acquisition system embedded in the DX-SAHP. The focus of this research is
the realization of a reliable Integrated Solar assisted heat Pump (DX-SAHP) capable to operate with maximum
energy performance indexes in order to allow the coverage of the user loads even under critical operating
conditions, like night or winter period, preserving all the operating and technical constraints (e.g. panel
frosting, overpressure for high temperature in the compressor).
Figure 2: (a) Preliminary experimental results obtained at following conditions: G = 200 W m-2,Tamb = 278.6 K,
Tuser = 303 K, ∆TH2O = 6.8 K. (b) Percent differences among values of enthalpy and condenser heat flux
calculated in differ ways.
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Nomenclature c specific heat, kJ kg-1 K-1 T environment temperature, K G lamp radiation, W m-2 T thermostatic bath temperature, K ℎ enthalpy, kJ kg-1 V compressor volume, m3 m refrigerant mass flow rate, kg s-1 ΔT 2 inlet – outlet water temperature, K P compressor power, W η isentropic efficiency, - P auxiliary power, W η volumetric efficiency, - q condenser heat flux, W ω compressor speed, rpm q evaporator heat flux, W density, kg m-3
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