Category Archives: EuroSun2008-3

Danish tests

The tests at DTI were done outdoors on a side by side configuration with three test panels of 3.5 m2 each and a single PV reference module. PV modules were each 190 Watt from Evergreen.

image004

System 1: SolarWall with two PV modules on top, variable flow (high)

System 2: SolarWall with two PV modules on top, variable flow (low)

System 3: SolarWall alone, variable flow (thermal reference)

System 4: PV module alone, natural ventilation (electrical reference)

The tests were carried out in open air at variable insolation and wind speed. Side-by-side testing was done to eliminate errors due to different ambient conditions. Nominal flow rates were:

PVT #1: 140 m[3]/h pr m2 PVT #2: 75 m3/h pr m2 T #3: 75 m3/h pr m2

Подпись: Figure 5. PV/T outdoor tests with 190 Watt Evergreen PV modules

The last PV panel #4 had an open back surface, typical of NOCT test conditions but not typical of an ordinary roof system.

Table 2 indicates the difference in air temperature between ambient and the air leaving the transpired solar panel. Irradiance, wind speed and flow rates are shown to all have an effect on the thermal gain.

Table 1. Test Conditions

Designated #

Test Type

Irradiance [W/m2]

Wind Speed [m/s]

1

PV — NOCT

810

1

2

Thermal

910

3

Table 2. Air Temperature Rise for various flow rates and PV modules.

Collector Type

108 m3/h. m2 6 CFM/FT2

108 m3/h. m2 6 CFM/FT2

36 m3/h. m2 2 CFM/FT2

36 m3/h. m2 2 CFM/FT2

1

2

1

2

[°C]

[°C]

[°C]

[°C]

BP PV/T

9

7

15

9

Evergreen PV/T

9

6

14

10

UniSolar PV/T

11

9

20

15

Bare SolarWall

16

13

28

20

Table 3. Electrical and thermal efficiencies and temperatures

Air

PV

PV n

PV n

T

Thermal

Thermal

Total

Test

Flow

Power

rated

Actual

avg

Power

n

T rise

n

Watts

C

Watts

C

EG

1

2

746

11.4%

10.3%

1878

23.2

14.41

32.4

EG

2

2

858

11.4%

10.5%

1276

14

9.74

23.4

BP

2

2

720

11.9%

10.5%

1220

13.4

9.36

21.3

BP

1

2

637

11.9%

10.4%

1954

24.1

15.12

31.9

US

1

2

477

6.1%

6.6%

2542

31.4

19.68

37.3

US

2

2

1962

21.5

15.06

N/A

US

2

6

557

6.1%

6.8%

41

3573

39.3

9.46

45.4

US

1

6

487

6.1%

6.7%

48

4076

50.4

10.8

56.4

BP

1

6

627

11.9%

10.3%

46

3545

43.8

9.42

51.6

BP

2

6

726

11.9%

10.5%

37

2534

27.8

6.72

35.8

EG

1

6

753

11.4%

10.3%

47

3229

39.6

8.56

48.8

EG

2

6

866

11.4%

10.6%

41

2443

26.7

6.43

36.2

The thermal results are interesting in that they clearly show reasonable heat gains and reasonable solar efficiencies. For comparison purposes, the thermal gains of the bare transpired SolarWall panel are shown in Table 2 from the previous certification test data on the same 10 m2 test panel.

Transpired collectors are installed for heating or preheating the ventilation air required in buildings. Most applications are designed to preheat air with a temperature rise in the 5°C to 20°C range. The test data shows that the temperature gain from the PV modules is between 6°C to 20°C or within the typical range for transpired collectors.

It was expected that the bare transpired collector would perform better than with PV modules mounted above it (PV/T). What is surprising is that by resizing the transpired collector or adjusting

the air flow rate, the same thermal design target with PV/T is possible as is currently produced in many transpired collector installations.

The NOCT tests are conducted with an open back and PV suppliers typically list a PV panel temperature in the 47°C range. The PV/T tests, (with a closed back) as listed in Table 3, which is typical of a BIPV configuration, show temperatures similar to or lower than the NOCT data.

The efficiency data for the PV portion of PV/T in Table 3 is based on the dimensions of the PV modules. The thermal efficiency data is based on the transpired collector area of 10 m2. Since the PV modules do not cover 100% of the transpired collector, the column showing Total n (efficiency) is not a straight addition of the PV and thermal numbers but is based on the gross test panel area of 10 m2.

Mathematical models for single components to be simulated

Most of the components to be simulated are already included in the TRNSYS standard library except for the cogeneration unit and the biomass boiler. Hence, new mathematical models have been appropriately written. These models include on one hand operating characteristic curves, on the other hand internal control logics. The characteristic curves have been derived by the elaboration of technical specifications acquired by various manufacturers and they refer to the following ranges of sizes: [70; 500] kWe for gas engine based cogeneration units and [15; 1000] kWth for biomass boilers.

Both the created models implement the following steps:

• Once the nominal heat power has been determined according to the subchapter 3.5, the model calculate left nominal features, e. g. the efficiency and the primary energy consumption at nominal condition;

• Once the mass flow and the temperature of the fluid entering each machine are given as input and a desired outlet temperature is selected, the models calculate the heat required for the stream to reach the set temperature. According to the selected control logic (discrete or continuous modulation), the model identifies the current load rate, the corresponding efficiency and primary energy consumption and the fluid outlet state.

For instance, the algorithm of the cogeneration unit is shortly described.

The first input required by the algorithm is the nominal heat power (Qreq, nom). The corresponding electrical power (Pnom) is determined by:

Pnom = 0.6148Q„ + 0.9282 [kW] (1)

The nominal electrical, thermal and first law efficiencies (ne, nom, nth, nom and respectively) are

calculated according to:

nenom = 0.0061Pnom + 31.284

[%]

(2)

Vhnom =-0.0133Pnom + 58.266

[%]

(3)

ninom =-0.0071Pnom + 89.51

[%]

(4)

The inlet mass flow and its temperature being known, the model calculates the heat required to heat up the stream to a previously selected temperature. Then, the transferred heat Qcog in [kW] is identified according to the internal control logic (heat load control and continuous modulation). The electrical power Ppart which is related to Qcog can be calculated through the following equation:

О P

^cog = 0.6685-^ + 0.3315 [%] (5)

OP

£^req, nom nom

Afterwards, the electrical efficiency ne, part in [%] corresponding to Ppart is determined according to:

Подпись: = -0.40
Подпись: e, part
Подпись: Пе image078 image079 Подпись: [%] Подпись: (6)

2

For the absorption chiller simulation, the model presented in [4] is used. The characterization of the chiller, in terms of distribution of the heat exchange area between the components and the typical internal flows, is made referring to a commercial chiller. The maximum relative error calculated comparing the model output to the manufacturer performance data does not exceed 10% and it is found for off design conditions (cooling water inlet at 35 °C).

SCADA System

A SCADA system (Supervisory Control And Data Acquisition) is implemented to monitor and supervise the tracking system.

A Supervisory Control and Data Acquisition (SCADA) System is used as an application development tool that enables system integrators to create sophisticated supervisory and control applications for a variety of technological domains, mainly in the industry field. The main feature of a SCADA system is its ability to communicate with control equipment in the field, through the PLC network. As the equipment is monitored and data is recorded, a SCADA application responds according to system logic requirements or operator requests.

In the developed supervisory system the SCADA application manages the overall system dynamics.

The Communication flux between the supervisory system and the control unit has been already illustrated in fig. 4.

Подпись: Fig. 4. Communication Strategy (SCADA - PLC Tracker)
Подпись: Fig. 5. Solar Park - PV panels

The SCADA PC is simultaneously a SCADA server and an internet server, as the implemented SCADA application is web enabled.

This application permits the selective access to the application depending on the user’s responsibility degree. In this paper we developed three user levels: Operators, Supervisors and Administrators. Among the main functionalities of a SCADA system there is the so called “Tag”.

A “Tag” is a defined variable that permits the change of information between the PLC network and the SCADA system, in a real-time environment. There are usually three types of Tags: PLC, Dummy and Compound. In the PLC Tags the PLC sets the variable values that are directly transferred to the Scada program. In the Dummy Tags the value is set by the user on the Scada interface and transferred to the PLC address. Finally Compound Tags are set by the Scada program, following the programmed operations.

3. Experimental Prototype

Simulation results

image056

The results of these simulations for the selected representative winter period from 16th — 31st July are shown in Figure 3. This graph shows the heating energy of the outlet air from the 16m2 PVT system simulated in TRNSYS compared with the required house heating as modelled using both IDA ICE and TRNSYS. Reasonable correlation is shown between the heating demand results for the house as simulated by IDA ICE and TRNSYS.

Figure 3 shows that on most days during the selected period the supplied PVT heating energy is in excess of the required house heating as modelled in both IDA ICE and TRNSYS. During the 4 day period where the required house heating is greater than that supplied by the PVT system it is expected that the indoor temperature would drop and the degree of this decrease can be seen in Figure 4.

Подпись: Fig. 4. TRNSYS simulation results for house temperatures with and without the PVT system for the period 16th - 31st July
The PVT system integrated into the house was modelled in TRNSYS to determine the resulting indoor zone temperatures when the heated outlet air from the PVT system was directed into the house. The PVT system operated on an input schedule and only delivered heating energy to the building during the colder months from May to August. The resulting indoor temperatures for the house with and without a PVT system are shown in comparison with the ambient air temperature in Figure 4. The running average temperatures for the house and ambient air were calculated by averaging the values over a 20 day period — 10 days before and 10 days after each time point.

Figure 4 illustrates that the heating energy supplied by the PVT system is effective in increasing the indoor air temperature of the well insulated house. The running averages show that the indoor temperature of the house with the PVT system is up to 8°C above the ambient running average temperature during the period from May to August and is also 3°C above the running temperature of the house without a PVT system.

The average running zone temperature presents a promising result in that the temperature does not drop below 20°C. However the simulated indoor house temperature results show that the indoor temperature can drop to a similar temperature level to the house without the PVT system, showing that there is very little heating input during this period. This was also shown in Figure 3. The temperature during this period can drop to 16°C overnight. Despite the thermal mass modelled in the house, the results showed greater than expected daily swings in internal building temperature. This shows that the coupling between the thermal mass and the indoor air could be improved. A method for doing this would be to increase the surface area of the thermal mass inside the zone by

adding thermally massive internal walls to the simulation model to facilitate the exchange of heat with the inside air.

Management Algorithm for cogeneration

image110

The high overall efficiency of cogeneration plants is a result of the simultaneous production of electricity and heat. This effect only is valid if both products can be used. The heat only can be used in the local heat system or stored for a later use. For that reason it is important to model the thermal system. To model the thermal system it is useful to transform the equations into a thermal equivalent circuit (Fig. 2).

The CHP can be seen as a current source QCHP which is modulated control variable ui(t). The fed thermal network is represented by a variable resistance Rnet, which is controlled by u2(t), and a capacity Cnet, which represents the thermal capacity by changing the temperature level in the allowed range. The usage of the thermal storage is necessary if Cnet is used completely and the CHP should operate anyway, e. g. in case of high feed-in tariffs. The switch Ssto, controlled by u3(t), represents the valve regulating the liquid flow through the storage. The storage is modelled as a capacity Csto and a parallel resistance Rsto = k 1Arepresenting the heat losses to the ambience. The thermal conductivity is given

image111 image112 Подпись: (1)

by kstoAsto. In this model all temperatures are referred to the ambient Tamb. This also can be written as differential equation:

The three input variables u1(t), u2(t), u3(t) define the output of system described before. It is assumed that Thot stays above 60 °C. For the simulation the thermal demand is given as a time series and defines u3(t). In the showcase it is not possible to modulate the CHP, so u1(t) can have the discrete values 0 and 1. To optimise the operation of the CHP the best relation of u1(t) and u3(t) according objective function must be evaluated. The target of the system operator is to minimise his operation costs. It is assumed that the CHP can fed into the electricity grid with a variable tariff cel(t) anytime. This directly influences the operation costs c(t) of the system. Neglecting heat storage losses, the operation costs depend on the feed-in tariff cel(t) and the fuel costs cfuel. The objective function can be defined as:

Jc(t) = J u Шшр [иеі -4elcd (t)]= mm (2)

t t nth

The used primary energy can be calculated with the thermal efficiency nth of the CHP. The variable nth/hei defines the ratio between produced heat and electricity. Nonlinear relations like the discrete operation of the CHP, the given time series for cel(t) and the thermal demand make an analytic solving very complex. To reduce the complexity we made linear approximations and transformed the problem to a mixed integer linear problem MILP [4]. These kinds of problems have got the advantage that their performance still with a high number of variables is very good. There are also efficient solvers with the Simplex algorithm available [5].

Test Results

3.1. Canadian Tests

The tests were performed over a two week period in October 2006 and the results are summarized in the following tables. Table 1 lists the two primary test conditions, #1 for NOCT test conditions typical of PV systems and #2 for test conditions typical of solar air collectors. The heat was drawn off at two rates, 36 and 108 m3/h. m2 (2 and 6 cfm/ft2) of collector surface and results are shown separately for each flow rate.

[2] Introduction

Hybrid Photovoltaic/Thermal (PV/T) collectors are devices that simultaneously convert solar energy into electricity and heat. The reason behind the concept of hybrid PV/T is only 6-15% (depending on the cell type and technology) of solar radiation converted into electricity by solar cell. The remaining energy is converted into heat which caused increased the solar cell temperature that will affect decreasing the solar cell efficiency. There are several reasons to combine Photovoltaic (PV) and Solar Thermal Collector into one PVT device which are larger overall conversion efficiency, reduced energy payback time, reduce economic payback time and improved aesthetics. Cooling the solar cell with working fluid like water and air will increase the solar cell efficiency. There are two ways to cool the PV/T, which are cooling by water and cooling by air. Cooling by water can be applied to produce hot water and cooling by air can be utilized for space heating application. Based on those facts, PV/T will be the ideal solution in the higher environmental temperature and also to keep long life the Solar Cell.

A Typical model of PV/T is the direct attachment of PV modules onto a solar collector surface. Per unit area the total efficiency of a PV/T panel is higher than the sum of the efficiencies of separate PV

[3] Rabl, A., (1985). Active Solar Collectors and Their Application, Oxford University. Press, Inc. New York

[4] Results and discussion

The results from HOMER show that for a load of 6000 kWh with a capacity shortage allowance of 10% and a hub height of 20 m not for all locations a feasible system is possible (Figure 3). For the load of 6000 kWh no feasible systems were found for the locations with the two highest latitudes. For loads of 3300 kWh and 1800kWh for all locations feasible systems were found. The NPC varies between $48,000 and $87,000 for the highest load and $17,000 and $33,000 for the lowest load. It is obvious that energy saving measures would a cost effective alternative instead of enlarging the system size.

[5] Introduction

The overall problem with the use of PV-systems is the high cost of the solar cells. This makes it appealing to concentrate irradiation on the PV module in order to minimise the required PV-area for the same output. With increased light concentration, there will be a demand of increased cooling on the PV cells in order to lower the working temperature preventing damages and maintaining cell efficiency. Solar8 is a photovoltaic/thermal parabolic concentrating system that tracks and concentrates light into a water cooled photovoltaic module working as a thermal absorber. By using the heat generated in the absorber, the photovoltaic/thermal device (PVT) generates not only electrical, but also thermal energy (Fig. 1). The photovoltaic module is formed by two sections, each one with 32 cells. These sections can be connected both in series and parallel. Generally, a concentrating system with a large number of series connected cells like Solar8 is highly sensitive of local defects in the optical system and on the solar cells, supposed to receive an equal amount of irradiation. The total electric

Demand side management

The applied approach splits the total load curves into shiftable and fix parts, where the shiftable power is assumed entirely redistributable over the day. When considering net production and demand profiles, net production can be represented as negative demand. The DSM strategy then refers to the more general case of valley-filling measures, leveling out demand sinks. Computationally, this means finding an optimal power level such that filled-in energy below this level equals the shiftable energy. This approach gives an upper level to the achievable load matching improvement through DSM.

Two cases (2a and 2b) are defined, where in the first case the shiftable demand is that from washing, drying and dishwashing appliances that could be easily rescheduled and involve relatively little user interaction. The second case involves all end uses except lighting, cold
appliances and undefined additional demand, and implies considerable violation of everyday routines.

2.2. Storage

A third option for increased load matching is introducing small storage units as buffers between the PV system and the distribution grid. To study the effect of such systems a simple battery model is applied. When the PV system overproduces power, the battery stores energy up to the battery capacity EUm. At net demand, the battery unloads energy with an efficiency kejf. State-of-charge limits for avoiding deep-charging are not included in the model. In the case studied here (case 3) a storage size EUm of 1 Wh/Wp is used and an efficiency kejf of 0.8.

3. Results

Electric and thermal output interaction

In Fig. 4, it is possible to comprehend the performance of a PVT hybrid system when it comes to thermal and electrical outputs interaction. As it is represented, when an electric load is connected to the electric circuit, electric power can be extracted. This means that part of the incoming irradiation is transformed into electricity by the PV cells instead of being absorbed by the thermal receiver. Hence, the thermal output decreases as much as the electrical output is extracted.

3.2. Reflector optical accuracy and design

Given that the measured system electrical efficiency (8.3% at 25°C) is significantly lower when compared with the bare cells efficiency (16% at 25°C), experiments were carried out in several components accuracy in order to estimate their influence in the final electric and thermal output breakdown. One of the most significant inaccuracies relates to the reflector. Ideally, every light beam perpendicularly incident to the glazed cover of the trough should be reflected to the PV module. Laser beam tests were carried out during the night and the glazed areas where the light was not focused on the PV cells were marked and are illustrated in Fig. 5. I-V curves were measured with and without the covers and the electrical output was roughly the same. The glazed marked area is approximately 15% of the total glazed area and can represent an optical efficiency margin of improvement on the reflector accuracy for future Simulation and calculation

Study on Thermal Performance of Hybrid Photovoltaic Thermal. (PV/T) Collectors with and without Electricity Generation

E. Yandri[2]* N. Miura1, T. Kawashima1, T. Fujisawa1, M. Yoshinaga2

1Solar Energy Research Group, Department of Vehicle System Engineering, Faculty of Creative Engineering,
Kanagawa Institute of Technology, 243-0292 Atsugi, Japan

2Department of Architecture, Faculty of Science and Technology, Meijo University, 468-8502 Nagoya, Japan

Corresponding Author, yandri@ctr. kanagawa-it. ac. jp

Abstract

A Hybrid Photovoltaic Thermal (PV/T) collector is an integration of photovoltaic and solar thermal technologies into one system. The idea behind the hybrid concept is that a photovoltaic (PV) cell converts only 6-15% of the incoming solar radiation into electricity and more than 85% of the solar energy is either reflected or converted into heat. PV/T is designed to produce both electricity and thermal energy simultaneously in order to give the highest solar energy conversion efficiency comparing with the separated system into photovoltaic panel and solar thermal collector with the same area. Many researchers have done a lot of efforts to improve its performance by experimentally and simulation analysis. This paper presents the analysis of experimental result for PV/T, focusing on the thermal efficiency with and without electricity generation. The experiment was done during the peak irradiation from 12.00 to 14.00, in the steady state condition. The flow rate is 4 l/min, the inlet water temperatures to the collector is 120C. Especially for this experiment, the result shows that the thermal efficiency of a PV/T is not changed significantly comparing with and without electricity generation. The heat loss for without electricity generation case is a bit higher than without electricity generation case which might influenced by the higher wind speed for without electricity generation case than with electricity generation case.

Keywords: PV/T, thermal efficiency, electricity generation, overall heat transfer coefficient

panels and solar thermal collectors. This is the ideal solution if the availability of the roof area is limited. The most common PV/T collector type for low to medium temperature applications follows the sheet and tube heat exchange design shown Fig.1, typically consist of PV module on the back of which and absorber plate (a heat extraction device) is attached. The purpose of the absorber is to cool the PV module which resulted improving the efficiency and to collect the thermal energy produced. The PV plate is fixed on to the absorber plate through a thin adhesive layer. The adhesive layer used is a material with good thermal conductivity, is resistant to extreme temperatures and is extremely improved electrical insulator.

Many researchers and institution have attempted to develop and evaluate the hybrid PV/T collector performance experimentally and analytically. Ito and Miura, experimentally and analytically studied the thermal performance of a PV/T collector that used a partially transparent PV module as a cover [2], [3]. Ito and Miura also reported that the collector efficiency was slightly less while generating power than while not generating power [4]. Othman et al. [5], studied theoretically and experimentally hybrid PV/T solar collector regarding its thermal and electrical performance, used air as a flowing fluid to extract heat from the photovoltaic cells and keep electrical efficiency of it a satisfactory level by the reduction of its operating temperature. The conclusion of this work this work is that is important to use fins as an integral part of the absorber surface in order to achieve meaningful efficiencies for both thermal and electrical output of the hybrid PV/T solar collector. Santbergen and Zolingen simulated various crystalline silicon solar cell configurations found that a standard untextured solar cell with a silver back contact has an absorption factor of only 74%. If a semi transparent solar cell is used in combination with second absorber the total absorption factor can increase to 87%, and if irradiance is absorbed in the back contact, the absorption factor can increase to 85%. They suggested to apply the rough interface in combination with a non standard metal as back contact [6].

The total efficiency of a PV/T npv/t is by adding the thernal efficiency and electrical efficiency Пе1 which can formulated as below

Подпись:npvt =Ъг +Пе1

The thermal effciency of a PV/T collector is defined as the rasio of the useful heat Qth [W] to the product of the aperture area Ac [m2] and the insolation I [W/m2], which is incident on the aperture [1]

Подпись: (2)Qth

AI

The useful heat Qth [W] is related to mass flow rate m [kg/s], spesific heat at constant pressure cp [kJ/kgoK], and inlet an outlet temperatures Tn [oK] and Tout [oK] [1]

Qth = mcp (tn — tout) (3)

The efficiency may depend on many factors; collector temperature, ambient temperature, insolation, flow rate, and the incident angle. In order to characterize a collector, one must therefore specify the conditions under which the efficiency has been measured or calculated. One would like to specify the conditions in such a way that the efficiency is defined unambiguously and can be measured reproducibly. This can be accomplished most easily if one bases the effciency on clear sky conditions.

The electrical effciency це1 of a PV/T collector is defined as the rasio of the power generated Qel [W]

to the product of the aperture area A [m2] and the insolation I [W/m2], which is incident on the aperture [1]

Подпись: (5)The the power generated Qel [W] is related to current Iel [ampere] and Voltage Vel [volt] generated by the solar cell [1]

Qei = IV

The common linear formula of the thermal efficiency is [7]:

Пп = Fn O-U ((,, — Ta )/I ] (6)

Подпись: m cP UA Подпись: f 1 - exp V image014 Подпись: (7)
Подпись: Fin

Where T is transmittance, a is absorptance, the product of та is the optical efficiency no, U is the collector heat loss coefficient, and I is irradiation. Fin is called the heat removal factor, defined by the equation [7]

image017 Подпись: (8)

Where Fm is called the collector efficiency factor that accounts to the resistant of absorber to the ambient Rabs amb and the resistant of front to ambient Rfr amb [7]

image019

b. Schematic experiment set up

image020 image021
image022
image023
Подпись: Oil Separator
Подпись: Solar Collector
Подпись: Manual Expansion 0Valve

Fig.1. PV/T and experiment set up

2.

Подпись: Fig.2a shows the comparison of thermal efficiency with irradiation I for with and without electricity generation cases. The thermal efficiency ijth of PV/T can be calculated from Eq.2 and Eq.3.

Result and Discussion

Fig.2. Thermal efficiency vs irradiation and ambient air temperature.

For with electricity generation case, the irradiation I is decreased approximately from 1000 W/m2 at 12.00 to 770 W/m2 at 14.00. For without electricity generation case, the irradiation I relatively steady approximately 1000 W/m2. The average thermal efficiency i)th for with electricity generation case and

image028without electricity generation case was relatively about the same. They are 0.5258 for with electricity generation case and 0.5268 for without electricity generation case. Fig.2b shows the comparison of thermal efficiency and ambient air temperature Ta. Although the irradiation I was decreased for with electricity generation case and steady for without electricity generation case, the ambient

temperatures Ta for both cases increased during those periods of time. During periods of 12.00 — 14.00, the average ambient temperature Ta was about 11.70C for with electricity generation case and 12.40C for without electricity generation.

Fig.3a shows the comparison of thermal efficiency nth with the average surface collector temperature Tcoll. The average surface collector temperature TcM for with electricity generation case was unsteady and smaller with average of 23.10C and 24.00C for without electricity generation case. Fig.3b shows the comparison graphs of the thermal efficiency rjth with the outlet water temperature Tout. The outlet

Подпись: a. Thermal efficiency and average surface collector temperatureПодпись:Подпись:image032water temperature for both with and without electricity generations cases were about the same in the period of 12.15 — 13.00, but from 13.00 — 14.00, they were relatively steady for without electricity generation case and decreased for with electricity generation case. It might be affected by the variation of the irradiation I. The average outlet water temperature Tout for with electricity generation case was about 15.9°C and 16.5°C for without electricity generation.

Подпись:image034b. Thermal and predicted electrical output Fig.4. Thermal efficiency vs irradiation and ambient air temperature As an effect of the trend of irradiation I, the thermal output Qth and electrical output Qet both with

electric generation also follow the same trend of irradiation I. It shows that the average thermal

output Qth for without electricity generation case relatively steady about 0.519 kW/m2 and decreased from 12.00 — 14.00 with average about 0.469 kWm2 for with electricity generation.

Fig.5a shows the scatter diagram of thermal output Qth vs (Tavg -Ta) /1 for with electricity generation

and without electricity. Fig.5b shows the scatter diagram of the thermal efficiency nth vs

image035
(Tavg — Ta) / (I — Qel) for with electricity generation and without electricity generation by excluding the

Подпись: . The total lossПодпись:Fig.6a shows the comparison of total loss Qloss tot and the heat loss to ambient Qloss amb of this analysis is calculated by

Qloss, tot = 1 (1 — nth -Vel )

The total loss Q, is the sum of the top loss Q, , back loss Q, , , and edge loss Q, , ,

loss, tot loss, top loss, back loss, edge ‘

and Eq.(9) can be expressed also

Qloss, tot Qloss, top + Qloss, back + Qloss, edge (10)

Подпись:

Подпись: loss,edge

To simplify the analysis, by assuming the ambient air temperature Ta and wind speed Vwind on the top and at the back of collector are about the same, and also neglected the edge loss Qt

image040 Подпись: (11) (11a) (11.b) (11.c)

focus to analyse the top loss Qloss t. Because of this experiment used on single cover PV/T, the top loss Qloss o involve are convection and radiation loss from absorber to cover and from cover to ambient and then the top loss Qloss tov can be formulated

Подпись:q — Ah (T — T

loss, cov-amb, conv icov conv cov amb

Where. Aabs and Acov are the absorber and the cover area (m2), s is the emissivity (dimensionless, 0.88 for glass [7]), d is the Stefan-Boltzmann constant (5.67×10-8 W/m2K4), Tcbsis the absorber temperature (K), Tcov is the glass cover temperature (K), and Tamb is the ambient air temperature and hcotw is the convective heat transfer coefficient from the cover to the ambient air as a function of wind

velocity Kind

Bay assuming Tsky — Tair — Tamb and Eq.(11c) and Eq.(11d) can be simplified [7]

Подпись: (12)hamb (Tcov Tamb ) hconv (Tcov Tamb ) + Scovd(Tcov Tamb )

Knb is the effective heat transfer coefficient for both convection and radiation which has value of 20W/m2 for 3m/s wind speed and 30W/m2 for 6m/s wind speed [7]. To simply analysis,

assuming the radiative and convective from absorber to cover ( Qtoss, abs-cov,.rad and Qloss, abs-cov, conv ) more

or less the same for with electricity generation and without electricity generation, and the analysis

focus on the radiative and convective loss from absorber to cover ( Qioss, cov-amb, rad and Qloss, cov-amb, conv X

Loss energy ratio (LER) is the ratio of total loss Qloss tot divided with irradiation I and formulated in

LER — Qosso /1 (13)

Fig.6a shows the average total loss Qiosstot for with electricity generation was about 0.309 kW/m2, and 0.467 kW/m2 for without electricity generation. The average loss from cover to the ambient with radiation and convection Qloss cov-amb was about 0.119 kW/m2 for without electricity generation and 0.075 kW/m2 for with electricity generation. The loss energy rasio was calculated from Eq.(13) and shows the trends similarly follow the trend of the total energy loss Qloss tot. The average loss energy ratio for with electricity generation was about 0.345 and 0.473 without electricity generation.

image044

 

Fig.6. Energy loss and wind speed

Fig.6b shows also the wind speed Vwind of with electricity generation was lower than the wind speed Vwind of without electricity generation. The average wind speed Vwind of with electricity generation was about 0.3 m/s and 1.1 for without electricity generation. Using Eq.(11.c), the radiative heat loss

from cover to the ambient Qloss cov-amb rad for without electricity generation relatively as same as with electricity generation. Using Eq.(11.d), the average convective heat loss from cover to the ambient Qioss cov-amb conv for without electricity generation was about 0.119 kW/m2, higher than the average

convective heat loss from cover to the ambient for with electricity generation was about 0.075 kW/m2. It proved that the wind speed has a significant effect to the ambient loss.

3. Conclusion

Especially for these experiments, some parameters, like irradiation, ambient temperature, were not so closed each other, they did not change significantly the thermal efficiency. It means that the thermal efficiency for with electricity generation is relatively as same as without electricity generation. The total loss Qloss especially for the heat loss to the ambient Q0ss cov-amb increased as same

as the wind speed increased. The analysis shows that the thermal efficiency formula in Eq.(6), (7) and (8) did not influence much to the parameters of collector heat loss coefficienct U, heat removal factor Fin, and collector efficiency factor Fm.

Acknowledgement

The support of “High-Tech Research Center Project for Private Universities: matching fund subsidy from MEXT, 2007-2011” for this research is appreciated.

References

[1] Duffie, J. A, Beckman, W. A., Solar Engineering of Thermal Processess (third edition), John Wiley & Sons, Inc. Hoboken, New Jersey

[2] Ito, S., Miura, N., Solar Air Collector Using Photovoltaic Modules as the Cover, Proceedings of ISES Solar World Congress, Budapest, Vol. 3, pp.271-276, 1993

[3] Ito, S., Miura., Usage of a DC Fan Together with Photovoltaic Modules in a Solar Air Heating System, Proceedings of ISES Solar World Congress, Gotheborg, Sweden, 2003.

[4] Ito, S., Miura, S., Performance of Photovoltaic and Thermal Hybrid Air Collectors, Proceedings of ISES Solar World Congress, Orlando, USA, Paper No. 1349, 2005

[5] Othman, M. Y., Yatim, B., Sopian, K., Bakar, M. N.A., Performance studies on a finned double­pass photovoltaic-thermal (PV/T) solar collector. Desalination 209 (2007) 43-49

[6] Santbergen, R., van Zoelingen, R. J.Ch., Modeling the thermal absorption factor of Photovoltaic / hermal Combi Panels. Heat Set 2005, Grenoble, France [3]

First results from TRNSYS simulations

Two decks have been created one for heating mode and one for cooling mode, but the entire year can be simulated all at once. A 10 minutes time step has been selected to promote the convergence of some components, in particular the controllers.

Not many simulations have been carried out up to now. However, some considerable results have come out which already addresses on the way to an optimal planning. In fact, during the first simulations the following aspects have emerged:

• the absorption chiller matches well the cooling peak demand but it often switches on/off at low loads

(Fig. 6.);

• the solar fraction is quite small: boilers are required to run the whole year;

• the tank serving the cogenerator is small: the engine switches on/off whenever low flows are extracted from the top of the tank.

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Please note: the on/off behaviour of the cogenerator and the absorption chiller is also due to the fact that thermal inertia effects are not included in both the mathematical models.

Fig. 6. Simulation of the cooling flow demand and the absorption chiller in a typical summer day.

2. Conclusions

SHC-CHP systems seem an interesting project solution but their planning requires a huge effort mainly due to the different behaviour of the solar collectors and cogeneration units. The procedure suggested in this paper is finalised to support the design of such plants in order that the mentioned components do not interfere in their respective operation. This procedure can be applied to whatever kind of building, its demand being known. Once the layout and the control strategy are fixed, the simulations address to the optimal solution in terms of size. The energy consumption corresponding to the final solution will then make clear whether and in which case this sort of project is energetically and economically convenient.

Acknowledgements

The authors would like to gratefully thank the STIFTUNG SUDTIROLER SPARKASSE for the financial support.

References

[1] Dorgan, C. B., Leight, S. P., Dorgan, C. E., 1995. Application Guide for Absorption Cooling/Refrigeration Using Recovered Heat. USA: ASHRAE.

[2] Petchers, N., edited by, 2003. Combined Heating, Cooling & Power Handbook: Technologies & Applications. Lilburn, GA: Fairmont Press.

[3] Troi, A., Filippi, H., Sparber, W., 2005. Practical Experience with Solar-Assisted Cooling in an Office and Educational Building in South Tyrol / Northern Italy. In: Otti, ed., 2005. 1st International Conference on Solar Air Conditioning. Germany, October 2005.

[4] Nurzia, G., 2008. “Design and simulation of solar absorption cooling systems”. PhD Thesis in Energy and Environmental Technologies, Department of Industrial Engineering, Bergamo University.