NOVEL HEAT PUMP SYSTEM FOR HIGHLY TRANSIENT PV OPERATION

Spinnler, Markus; Hörth, Leonhard; Böing, Felix; Wolf, Stephan; Sattelmayer, Thomas Technische Universität München, Lehrstuhl für Thermodynamik

Boltzmannstraße 15, 85748 Garching

ABSTRACT: Against the background of reduced PV feed-in tariffs in many important European countries and increasing problems with grid stability at PV peak power times due to tremendously growing capacities, electricity storage and self-consumption of PV power has emerged to be a crucial topic for the PV community. One promising approach lies in thermal storage of PV power in the HVAC supply for buildings. Besides cost-effective but low-efficiency ohmic heaters, a new development is compression heat pumps and chillers directly operated with PV. However, if directly coupled to a PV-plant, commercially available heat pump systems suffer from severe control problems due to highly transient PV power supply. Both On-Off control and combinations of scroll compressor and frequency inverter have a more or less limited control range. In the present study, a novel heat pump set-up with swash-plate compressor will be presented, offering a number of advantages: other than with conventional single-degree-of-freedom on-off or FI-scroll control systems, rpm plus cylinder stroke can be actively controlled. At frequently occurring solar part load conditions coming mostly parallel with elevated heating demand, swash-plate compressors offer a higher efficiency factor at a much wider control range. The study is providing details on the novel heat pump set up with swash-plate compressor together with experimental results on part-load efficiency as well as step function response on sudden PV power increase or decrease. Based on these results, theoretical investigations yield a first potential analysis of the novel system compared to a conventional FI-scroll compressor set-up. A yearly simulation showed improvement potentials of 35 % in PV self-consumption and 30 % in solar heat supply. Keywords: Heat Pump, Inverter, Scroll, Swash-Plate, PV

1 INTRODUCTION 1.1 Motivation of the Study

The development of both compression heat pumps and compression chillers directly driven by photovoltaic (PV) power has to be seen against the background of two increasingly severe problems in Europe: 1st, in many European countries, financial promotion of photovoltaic energy, mostly through feed-in-tariffs regulated by law, is being reduced so that self-consumption of PV power is becoming more and more attractive. While for example in Germany, feed-in tariffs for a small-scale (< 10 kWp) domestic PV-plant have been as low as 12.69 €Ct/kWh in September 2014 [1], the average electrical energy price for private customers was 29.40 €Ct/kWh [2]. 2nd, in a much larger context, increasing problems with grid stability due to tremendously growing installed PV capacities occur at PV peak power times.

For these reasons, new possibilities to increase self-consumption, mainly in domestic application but also a region-wide increase in (electrical) storage systems have emerged to be crucial topics for the PV community. Even though a vast range of battery storage systems has entered the market during the last two years, especially for small-scale domestic energy storage in the 5 kWh capacity range, economic feasibility is hardly achieved. [3]. Thus, one promising approach lies in thermal storage of PV power in the Heating, Ventilation and Air Conditioning (HVAC) supply for Buildings.

While the majority of recent studies concentrate on resistive (ohmic) electrical heating for grid stabilization and optimized self-consumption [4-5], first conside-rations to include controllable compression heat pumps and –chillers emerge in research as well as in the heat pump industry. Especially in part-load times (winter and transition times), when solar irradiation and thus PV power is low while heating loads are high, heat pumps offer substantial advantages compared to ohmic heaters

due to their high efficiency (Coefficient of Performance, COP). Hence, the advantage of a high PV coverage goes hand in hand with an optimized solar heat supply. Due to the developments in PV prices, also for solar cooling, the option of coupling PV with bifunctional compression chillers/heat pumps becomes more and more attractive.

However, when directly coupled to a PV-plant, commercially available heat pump systems suffer from severe control problems due to the highly transient PV power supply, see figure 1. On cloudy days, solar irradiation values vary between 100 W/m2 and – at arri-ving or passing cloud fronts – up to 1300 W/m2 in time-scales of less than 20 seconds, which are severe conditions for heat pump control.

Figure 1: Example of Global Horizontal Solar Irradiation during a Sunny and a Cloudy Day

State-of-the-art heat pumps are either equipped with a simple on-off control or – for future, more advanced inverter heat pumps – a combination of scroll compressor and frequency inverter (FI) to control refrigerant mass flow and thus heating power via the compressor driving

speed (rpm) [6]. While for on-off-systems, it is nearly impossible to realize a direct PV supply, the main disadvantage of converter heat pumps still is a limited FI control range, which covers only 40 – 100 % of the nominal power. Furthermore, under lower part load, scroll compressors have an extremely reduced efficiency and limitations in durability. For these reasons, novel heat pump solutions with highly flexible control are of utmost importance for tackling the problem of an optimized usage of transient PV power without high-capacity battery storage.

1.2 Scope of Work

Even though they do not yet dominate the market, inverter heat pumps have many advantages compared to on-off systems and thus shall be the benchmark for the present study. Additionally, a novel heat pump set-up with swash-plate compressor will be presented, offering a number of advantages: instead of the conventional single-degree-of-freedom control system with FI and scroll compressor, now rpm plus cylinder stroke can be actively controlled. At frequently occurring (solar) part load conditions, swash-plate compressors basically offer a higher efficiency and a much higher control range. Furthermore, the system can be switched from heat pump to compression chiller mode resulting in considerable self-consumption under wintry and summery conditions. However, for the sake of brevity, this paper will only focus on heat pump operation.

The new system will be investigated for a small power range focusing on domestic application – this means starting from a PV-plant of about 2.5 kWp, resulting in a heating power of approximately 5 kW and higher.

The scope of the present study is 1st to prove feasibility of a directly PV-driven heat pump and chiller equipped with a swash-plate compressor under the indicated PV power of maximally 2.5 kWp. 2nd, to present first data on part load efficiency of the novel set-up and 3rd to deliver data on PV power step function response, as it would occur in case of appearing or passing clouds. 4th, all data will be compared with a similar, but conventional heat pump with FI-controlled scroll compressor. A 1-D simulation model will assess the potentials of the novel set-up. 2 DESIGN OF THE EXPERIMENTAL APPARATUS

As already indicated in section 1, the aim of the study is to evaluate controllability of two different compressor configurations. The experimental test facility was designed accordingly.

Considering conventional (PV-driven) heat pumps with optional chiller mode, which are already commercially available, the benchmark for building up the test rig was following for example the “WPL-/WPF-cool” series by Stiebel Eltron [7-8], “Vitocal 300-G/350-G” by Viessmann [9] or “HP10MR” by Sonnenkraft [10]. 2.1 Refrigerants

During the last years, a great deal of discussion about the choice of refrigerants was coming up. Along with favorable thermodynamic properties like high evaporation enthalpy, flat vapor pressure curve, low pressure difference between evaporation and condensation pressure, to mention only the most important specifications, modern refrigerants target on environmental safety, which means they have to have a low Ozone Depletion Potential (ODP) and lately also low

Global Warming Potential (GWP). A further topic is their toxicity and flammability, the latter of minor significance for stationary applications.

While refrigerants with ODP > 0 are prohibited since the Montreal Protocol 1989, stationary chillers with GWP > 2200 and split-AC devices with GWP > 750 will be banned starting from 2020 [11].

In small-scale and domestic heat pumps, refrigerants R134a, which is very common in mobile air-conditioners for cars, as well as R290, R407C [7-8], R410A [9-10] are well established. Table I gives a summary of the discussed refrigerants, which are all non-toxic and, except R290 (propane), non-flammable.

As can be seen, all refrigerants in question have a rather high GWP in the critical range of [11] except R290 which is flammable. Alternative refrigerants like R1234yf (similar properties to R134a, GWP = 3, flammable) R744 (CO2, transcritical process at high pressures), R717 (NH3, toxic), R718 (H2O, high boiling temperature) are being investigated, but not yet esta-blished for the indicated reasons. Table I: Established Refrigerants for small-scale and stationary heat pumps and chillers. Refrigerant R134a R290 R407C R410A Substance Tetrafluor- Propane Mixture Mixture Ethane Formula CH3-F-CF3 C3H8 Boiling Temp. -26,2°C -42,0°C -44,3°C -51,5°C ODP 0 0 0 0 GWP 1.300 3 1.652 1.980

As already said, the focus of the current study was set

on a comparison between conventional “inverter heat pumps”, with FI/scroll compressors and with swash-plate compressors. Thus, compressor types available on the market had to be applied which mostly are designed for operation with R134a and its mixtures like R407C, R410A. Swash-plate compressors up to now are nearly exclusively used for mobile applications, where R134a is state of the art with a possible future development towards R1234yf, enabling the use of practically the same compressors. R290 technically would be a good solution as well, but due to its flammability it is seldom used in the home appliances market.

As R134a is widely applied and full data sheets for scroll and swash-plate compressors, as well as cooling cycle components are available, the choice for the current experimental test rig was R134a. A variation of refrigerants is planned to be subject of future investigations.

2.2 Compressor types under consideration

Most of the installed domestic heat pumps, also those offered for direct PV-operation, are equipped with scroll compressors. The name “scroll” is standing for an involute spiral, which is attached to an equivalent spiral casing, see figure 2 left side. By a translatoric, circular movement of one spiral, several gas volumes are formed, which are decreasing during the progress of motion.

New in the current study is the application of a swash-plate compressor, which belongs to the family of axial piston compressors, see figure 2 right side, and is mostly applied in air conditioning systems of vehicles. The rotational axis is actuated by a V-belt, in mobile applications by the combustion engine, in the present application by an electric motor.

The tilt of the swash-plate, which is fixed to the rotational axis, leads to an axial movement of the cylinders, the stroke. The tilt can be controlled hydraulically or electromechanically, thus controlling the cylinder stroke. The main advantage of using swash-plate compressors in stationary heat pumps is this second degree freedom in control, as will be discussed in the next section, their applicability at high rpm, their huge control range and theoretically a higher efficiency at part-load conditions. One disadvantage lies in a slightly lower efficiency at full load conditions. [12]

Figure 2: Left: Operation Principle of a Scroll Compressor [13, p. 405], Right: Side View of a Swash-Plate Compressor [14].

2.3 Compressor control modes As already said, one of the main problems in directly

PV-driven heat pumps is the controllability of the process. i.e. the heating power as a function of the refrigerant mass flow through the compressor as a function of solar irradiation. For the time being, two control solutions, both in combination with a scroll compressor are established:

(1) On-off control with a conventional scroll compressor (O-I-scroll), which leads to a short operation time under nominal conditions, causing high run-up losses and immense direct losses at solar partial load conditions. As the compressor needs several minutes of overrun for shutting down, direct PV operation under these conditions is hardly feasible. However, for most of the available systems on the market, also those offered for direct PV operation, O-I-scroll still is state-of-the-art [15].

(2) Drive control of a conventional scroll compressor with a frequency inverter (FI-scroll), allowing to adapt the number of revolutions (rpm) and thus the refrigerant mass flow through the compressor. These types of heat pumps are called “Inverter Heat Pumps” which are being investigated by some manufacturers and are, to some extent, already entering the PV equipment market.

While FI-scroll has large advantages compared to I-O-scroll, there remain still some disadvantages: 1st, the self-consumption of the associated power electronics is reducing the Coefficient of Performance (COP) of the heat pumps, 2nd, the control range of the scroll compressor is only going down to 40% of the nominal power due to a heavy starting torque and lubrication problems at low rpm [15]. This still leads to problems at low solar irradiation conditions – unfortunately mostly parallel to high heating loads. 3rd, their low volumetric efficiency of the compressor at low rpm at low solar irradiation conditions is suboptimal [16].

For these reasons, the studied control option seems to be promising, especially at part load – i.e. winter, low solar irradiation, high heating load: (3) Swash-plate compressor (Swash) with controllable rpm plus cylinder stroke, which offers two degrees of freedom in control and a wider control range of 5% - 100% nominal

power [15]. Table II is summarizing the control options for heat pumps in direct PV operation without battery storage.

Table II: Control Strategies for PV-driven Heat Pumps Control Mode I-O-scroll FI-scroll Swash Control Range 100% 40-100% 5-100% Deg. of Freedom On-Off rpm rpm / stroke Remarks Cuts off limited range, lower efficiency most of low partload at high solar solar load efficiency. load.

The main advantage of applying swash-plate

compressors for stationary heat pumps is the second degree of freedom in control: while FI-scroll is to be controlled exclusively via rpm, swash-plates can run at constant grid frequency and the cylinder stroke can be controlled smoothly with a higher control range. The second option would be to control both rpm and cylinder stroke. Both options were realized in the test rig, while for the present study, only a variation of stroke was investigated.

2.4 Set-up of the test rig Test Rig Architecture

As it is planned to run the test rig both in the laboratory with an artificial power source to simulate PV power input as well as on the outdoor test center of the institute, connected to a real PV plant, the rig was installed on a transportable trolley, see figure 3. The plant is divided in two sections: (1) the compressor test line (fig. 3, left compartment), where the compressors and their respective drive trains and FIs are installed, and (2) the interface line with condenser, evaporator, fluid collector and demister as well as expansion and switching valves (fig. 3, right compartment).

Figure 3: Architecture of the Test Rig – Compressor Line (Left) and Interface Line (Right).

Compressor Test Line Figure 4 is showing a scheme of the test rig. As was

already highlighted, the main focus of the experiments is a comparative study of two control strategies: FI-scroll and Swash. Thus, the hydraulic set-up comprises two compressor test benches in parallel, which can be directly hooked up alternatively.

For the FI-scroll option, an Emerson Copeland ZH21KE-TFD scroll compressor with 5.4 kW heating power at evaporation temperature 0°C, condensation temperature 50°C (R134a, superheating 7 K, subcooling 4 K) was installed. The compressor with a fix displacement volume of 45 cm³ is directly driven by a refrigerant cooled electric AC-motor coupled with an external FI.

For the swash-plate option, a Denso 6SEU14C with 6 cylinders and a maximal displacement volume of 140 cm3 was chosen. The swash-plate compressor is driven by an external air cooled electric AC-motor (Dema EM 3000 STE), which can optionally be rpm-controlled with a FI. The nominal driving power is 3000 W, which gives, at a COP of 3, a heating power of up to 9 kW. Due to a lack in availability of compressors, it was not possible to compare FI-scroll and swash options at exactly the same heating power range. Thus, the swash-plate compressor is working under even lower part-load, which leads to an underestimation of its potentials in this study. Interface Line

As generally, the water/water heat pump configu-ration has the advantage of higher efficiency over air/water heat pumps, the current design was realized as a water/water system. Thus, both condenser and evaporator are water cooled or heated, respectively. For being able to switch from heat pump to chiller mode, both heat exchangers are identical SWEP B8THx30 with 6.6 kW evaporation and 13 kW condensation power. Both are heated or cooled with laboratory thermostats.

Figure 4: Process Flow Diagram of the Entire Test Rig with Heat Sink and Source.

For the laboratory tests, transient irradiation and thus transient PV power is simulated with an artificial AC power source at 50 Hz 400 V DC/AC-inverter level.

The evaporator outlet superheating temperature is con-trolled via the electronic expansion valve (EEV). For the current application, an EEV Danfoss ETS-14 with electric step motor adjustment was installed. For switching between heat pump and chiller mode, the system is equipped with a 4-way reversing valve, however, chiller operation of the plant is not part of the present study. Measurement Devices

The test rig is fully equipped with temperature, pressure and mass flow sensors to determine heating/-cooling power and COP in transient operation. In addi-tion, data acquisition hard- and software from National Instruments are used to record the measured data. 4 EXPERIMENTAL RESULTS 4.1 Time Scales for FI-Scroll and Swash

As an important issue for the feasibility of a direct PV-driven compressor, the response times of both compressor types at highly transient irradiation are

investigated. Figure 5 gives the electrical compressor (black) and heating power response (grey) to a control signal (blue) step function for the Swash configuration. As can be seen, 1.7 kW steady-state compressor power at constant rpm of 50 Hz can be achieved within 10 seconds starting from almost 0 % relative displacement volume sV,rel, which roughly corresponds to the time scales of a passing cloud. A lag of 5 s between maximum heating and compressor power occurs due a small delay in the control step from 0 % to 60 % displacement and due to thermal capacities in the system. Heating power of roughly 4 kW settles down 50 s after compressor power which is due to the heat capacity of the fluids in condenser and secondary cooling circuit.

Figure 5: Step Function Response for the Swash-Plate Compressor at a Source Temperature of 0°C, Constant Compressor Speed of 50 Hz at Variable Displacement Volume.

Figure 6: Step Function Response for the Scroll Compressor at a Source Temperature of 0°C at Variable rpm.

Also aiming for 4 kW heating power, a correspond-ding plot for FI-Scroll is given in figure 6 with a compressor power of 1.2 kW. Here, the motor speed nrps is increased from 0 Hz to a nominal 50 Hz within

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1 second. It can be seen, that the time scales and delays show only negligible differences to the swash-plate configuration. Thus, it can be said, that from a response-time point of view, both configurations are equally suitable for highly transient heat pump operation.

While swash-plate compressors usually have a higher part-load efficiency, the situation in the current study is different: as can also be seen in figs. 5 and 6, FI-Scroll reaches a COP = Qheat/Pcomp of 3.7 at steady-state, where-as Swash achieves only COP = 2.7. This is due to two reasons: 1st, in the current configuration, the dimensions of the swash-plate compressor are not optimal: the nominal heating power of the swash-plate configuration lies at 9 kW compared to 5.4 kW with the FI-scroll system – in other words, Swash is operating at even lower part-load conditions (about 60 % nominal power at 5.4 kW) which means that Swash potentials are systematically underestimated in the current study. 2nd, FI-Scroll works with an integrated electric motor which is cooled by the refrigerant and thus, in heat pump mode, makes use of the waste heat. The swash compressor is driven externally. 4.2 Part-Load Ability Until Start-Up and Shut-Off

As already discussed, the most essential topic for the current paper is the part-load capability of both FI-Scroll and swash-plate configurations, and here especially the lower power limits for compressor start-up and shut-down. To evaluate these limits, increasing and decreasing power ramps from 0 % to 100 % of the nominal heating power were investigated experimentally.

Figure 7: Control Signal Ramp for the FI-Scroll Compressor Set-Up – Start-Up- and Shut-Down Limits.

Figure 7 shows the up and down power ramp for FI-Scroll, using the compressor revolution speed (blue curve, right axis) as control variable. As in preliminary tests, it was found out that the power limit for FI-Scroll lies somewhere near 30 % of the nominal power and in order to prevent damages on the equipment, the initial compressor revolution speed was set to 20 Hz, which roughly corresponds to 25 % nominal power.

It can be seen, that initially, high compressor power Pcomp (black) at zero heating power Qheat (grey) occurs due to idle consumption of the electric motor. As at t = 60 s, Pcomp is coming down and Qheat starts increasing – this can be identified as start-up point, corresponding to a part-load of 37 %. This value can be confirmed at shut-down t = 410 s, where the compressor again is going idle (Pcomp rising) and Qheat falls to 0 kW. Note that during the down-ramp, the delay between Pcomp and Qheat is due to thermal

capacities and due to the use of motor waste heat in the heat-pump process.

Accordingly, figure 8 shows the ramp curve results with the Swash configuration, varying relative displacement at constant rpm of 50 Hz. As it was expected, that here, power limits are much lower, the ramp was started at 0 % relative displacement volume at a constant rpm of 50 Hz. As oscillations in tilt control cause a light delay and overshooting during start-up, maybe also enhanced due a certain starting torque on the high-pressure side of the heat pump, only the down-ramp yields reliable results. It can be seen, that Pcomp as well as Qheat smoothly follow the control signal until at t = 510 s, compressor power vanishes. This point corresponds to a part-load capability of roughly 7 %. It can also be seen, that the displacement volume sV,rel and therefore both compressor and heating power Pcomp and Qheat are not linear to the control signal.

Figure 8: Control Signal Ramp for the Swash-Plate Compressor Set-Up – Start-Up- and Shut-Down Limits.

5 NUMERICAL INVESTIGATION 5.1 System Model

In the following, a potential analysis for the two heat pump configurations coupled to a PV plant shall be performed based on a 1-D simulation model. For the PV system, a peak power of 2.5 kWp with optimal orientation for Central Europe (30° tilt angle facing South) is assumed, which is a typical size for small single-family homes. In a first step, the effective PV power is calculated hourly based on the test reference year (TRY) of Munich, Germany (48.14° N, 11.58° E). Depending on the respective sun position, the horizontal diffuse and direct TRY irradiance data are converted to the PV system’s tilt angle, assuming diffuse irradiance to be isotropic [19].

The applied solar cells (BOSCH M 3BB C4 1200 with STC-efficiency ηSTC = 18.53 %) are modeled using a modified one-diode approach as described by WALKER [20]. In order to determine the slope of the I-V-curve at open-circuit voltage, an approximation function by WAGNER [21] is used. The cell temperature is described by an empirical model of LASNIER and ANG [18] including ambient temperature and irradiance.

The aim of the current study is the behavior of different compressor configurations under solar irradiation regardless of the domestic HVAC equipment behind it. Therefore, several simplifications have to be made: 1st, the heat pump is modeled quasi-stationary with

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the assumptions of isobaric evaporation and condensation at constant superheating ΔTsh and subcooling ΔTsc. The condensation temperature Tc is set to be constant and the evaporation temperature Tevap is calculated depending on TRY ambient temperature. This leads to the assumption that, at any time, the generated heat is completely consumed at a constant sink temperature. Following the results on part-load capability, lower power limits for FI-Scroll were set to 30 % nominal power, whereas Swash operates down to 10 %. Table III shows the basic boundary conditions for these calculations. Table III: Boundary Conditions for the Numerical Model PV Orientation Location PPV,p nrps,swash South, 30° Munich 2.5 kW 30 1/s ΔTsh ΔTsc Tc PPV,low (Scroll) PPV,low (Swash) 7 K 10 K 55°C 30 % 10 %

The irreversible compressor model uses the approach of isentropic compression by calculation of the isentropic and volumetric efficiency ηisen and λvol.

𝜂!"#$ =ℎc,  out − ℎc,  in

ℎc,    isen,  out − ℎc,in

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𝑉swash ∙ 𝑛rps ∙ 𝑠V,rel ∙ 𝜌c,  in (3)

These performance figures primarily depend on the

pressure ratio and the rotary speed nrps (FI-Scroll) or the relative displacement sv,rel (Swash). To validate the model, the experimental results shown in section 4 were used to fit the polynomial approach of DIN EN 12900 [22]. Power control under part load conditions is implemented as a variable relative displacement of the swash-plate compressor (sv,rel = 0,1…1) and a variable rpm of the scroll compressor (nrps = 30…75 Hz). Auxi-liary units, such as pumps, fans or inverters are neglected.

In order to evaluate the two individual heat pump configurations, further performance figures need to be defined. Typically, the Heating Seasonal Performance Factor (HSPF) is used to label different heat pump systems. In the present case, a utilization factor ηPV, and a performance factor εPV should be used, both relating to the generated PV power over one year.

𝐻𝑆𝑃𝐹 =𝑄heat

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In the case of PV-driven heat pumps, ηPV depicts, how

much self consumption can be achieved by the heating

application, which is a crucial parameter for evaluating system economics. εPV stands for yearly heating energy provided through PV power and should be lower than conventional HSPF due to frequent part-load operation.

5.2 Numerical Results

In figure 9, the hourly (blue) and weekly (black) averaged PV power output over one year can be seen together with the weekly utilization of FI-Scroll (red) and Swash (green) compressor types. The plot shows clearly, that the annual utilization of FI-Scroll is lower than that of the swash-plate system.

Figure 9: Generated PV Power and Weekly Averaged Utilization of the 2 Compressor Types over One Year.

Integrated over one year, the performance numbers introduced in equations (4) to (6) show a clear picture: As had to be expected from the experimental results, HSPF of the scroll compressor (HSPFscroll = 3.9) is explicitly higher than that of the swash-plate (HSPFswash = 3.0), which is only the case for the compressors used in the current study, see section 4.1. However, when related to the PV power, a completely different situation is arising: both utilization factor ηPV and performance factor εPV of the Swash configuration are higher than that of FI-Scroll. Table IV: Utilization Factor ηPV and Performance Factor εPV for a Yearly Simulation. Swash-Plate FI-Scroll Improvement ηPV =0.97 ηPV =0.63 35 % εPV = 3.57 εPV = 2.48 30 %

Figure 10: Generated PV Power and Utilization of the Two Compressor Types on a Sunny Day.

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As can be seen in table IV, the utilization factor ηPV, which is depicting the potential for self-consumption of PV power and thus is an important economic parameter, is roughly 35 % higher with the swash-plate compressor than with FI-Scroll. Even the yearly performance factor εPV is about 30 % higher with Swash, which means, that the higher isentropic and volumetric efficiency of the currently used scroll compressor, see the HSPF results, is outperformed by the higher part-load capability of the swash-plate compressor.

Figure 10 illustrates this effect: the modeled output for a sunny summer day in Munich clearly shows the differences in the start-up-points of the different compressor types. Swash has a considerably longer operation time than FI-Scroll, resulting in a much higher utilization ηPV and, even at reduced efficiency, also higher performance factor εPV, see table V.

Table V: Utilization Factor ηPV and Performance Factor εPV for a daily Simulation on a Sunny Day. Swash-Plate FI-Scroll Improvement ηPV =0.98 ηPV =0.81 17 % εPV = 4.63 εPV = 4.11 11 %

As can be expected, this behavior is much stronger on a cloudy day with a high transition of low part-load conditions, as shown in figure 11. Table VI shows the results for the corresponding utilization and performance factors. It can be seen, that under weak irradiation conditions, the potentials of the swash-plate compressor increase by 200 % compared to a sunny day.

Figure 11: Generated PV Power and Utilization of the 2 Compressor Types on a Cloudy Day.

Again, the reason for this behavior clearly lies in the much better part-load ability of the swash-plate compressor. Even though Swash COP as a function of PV power is widely lower than COP for FI-Scroll, coverage of the lower part-loads from 30 % down to 10 % is a clear advantage in overall self consumption ηPV and PV heat supply εPV.

Table VI: Utilization Factor ηPV and Performance Factor εPV for a daily Simulation on a Cloudy Day. Swash-Plate FI-Scroll Improvement ηPV =0.94 ηPV =0.66 30 % εPV = 4.34 εPV = 3.35 23 %

6 CONCLUSIONS

In the present study, a novel control system for stationary, directly PV-driven compression heat pump and chiller systems in the 2.5 kWp domestic applications class was demonstrated and compared with a conven-tional scroll compressor with FI control. Heat pumps with swash-plate compressors offer a high control range at extreme part load conditions lower than 10% of the nominal power together with a fast step function response at highly transient solar irradiation, for instance on a cloudy day. Response times of approximately 5 seconds following a PV power step function were successfully demonstrated for both FI-Scroll and Swash-Plate configurations, proving the feasibility of PV-driven inverter heat pumps as well as the novel configuration. Performing ramp tests, part-load capability of the FI-scroll compressor was determined to be 37 % minimum operation power, whereas the swash-plate configuration can still be operated at 7 % nominal power.

Especially in winter or in transition time, when disadvantageous weather conditions come along with high heating loads, this is clearly an advantage: compared to a sunny day, efficiency potentials of the swash-plate compressor are 200 % higher on a cloudy day. In a simulation over one year, it was found out, that the newly defined utilization factor ηPV, which expresses the potential for self-consumption of the PV power, can be increased by 35 %, whereas the performance factor εPV of heating power related to PV power could be enhanced by 30 %.

Future studies should aim at comparative compressor data with similar heating power, as with the currently used swash-plate compressor, potentials were systema-tically underestimated. Some minor problems with heat pump control during start-up need to be addressed. Furthermore, it will be crucial to couple an HVAC building model to the current heat pump model for enabling economic considerations. In a later step, investigations towards the development of solar cooling devices with switchable compression heat pumps/chillers and a choice of variable refrigerants has to be undertaken. ACKNOWLEDGEMENTS

The authors gratefully thank “Solarenergie-förderverein Bayern e.V.” for financial support within the “PVCool” research project. REFERENCES [1] Gesetz für den Vorrang Erneuerbarer Energien

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