Солнечная и другая альтернативная энергия

Портал о солнечной и другой современной альтернативной энергии. Солнечные батареи, ветровые генераторы, батарейки, аккумуляторы, современные элементы питания и современные способы зарядки. More »

Солнечная и другая альтернативная энергия

Портал о солнечной и другой современной альтернативной энергии. Солнечные батареи, ветровые генераторы, батарейки, аккумуляторы, современные элементы питания и современные способы зарядки. More »

Солнечная и другая альтернативная энергия

Портал о солнечной и другой современной альтернативной энергии. Солнечные батареи, ветровые генераторы, батарейки, аккумуляторы, современные элементы питания и современные способы зарядки. More »

Солнечная и другая альтернативная энергия

Портал о солнечной и другой современной альтернативной энергии. Солнечные батареи, ветровые генераторы, батарейки, аккумуляторы, современные элементы питания и современные способы зарядки. More »

Солнечная и другая альтернативная энергия

Портал о солнечной и другой современной альтернативной энергии. Солнечные батареи, ветровые генераторы, батарейки, аккумуляторы, современные элементы питания и современные способы зарядки. More »

 

Energy transfer when pouring

The energy of the hot water poured by the customer is defined as:

Ethw(i) = mthw(i) • CP • (Tthw — Tcw) (1 1)

Подпись: E(i + 1,n) Подпись: 1 - Ethw(0 E(i n + a -1) + Ethw(l) • E(i,n + a) - E(i,n) E(i,a)) E(i,a) Подпись: (12)

Where thw stands for tap hot water and cw is cold water. The quantity of hot water poured from the tank during one time step can be higher or lower than the mass of water contained in one element. Never the less, the next relationship for the energy of an element n stands.

Where a is the number of elements providing the hot water for the pouring, 1 or higher. If n+a is larger then the total number of elements, N, than cold water is added to the bottom elements.

3.2 Model validation

To valid our numerical model a comparison is done with a commercial solar thermal software TSol and with experimental data. The computation time step p is one hour. Identical systems are compared in both cases located in two cities: Paris and Grenoble. The present model over estimates the performance of the system with 1.4% in Paris and 1.3% in Grenoble when comparing with TSol, and with less than 3% in both cities when comparing with experimental data.

4. Results and comments

Energy input from the solar loop

The solar heat exchanger is a cupper coil immerged at the bottom of the tank. One or more elements contain this exchanger as the user can decide. We consider that the heat provided by the

4

solar loop is transferred in a time step to one or more elements depending of their temperature. For example, if the last three elements have a lower temperature than the hot water produced by the solar exchanger than only those three elements will be heated. No influence is considered for the fourth element, during that time step.

The solar panel temperature is computed depending on the operation of the pump: if the pump is not running than

Tpanel(i + 1) = Tpanel(i) + •

panel

• Esun(i + [ • P-K1 • (Tpanel(i) — T0(i + 1))-K2 • (Tpanel(i) — T0(i + 1 ))2

image225 Подпись: (9)

And if the pump is running than

Where mpanel stands for the mass of water contained by the panels in kg, msolar stands for the water quantity (kg) flowing through the solar loop in a time step p, S panel is the active surface of the panels, Tho and Thi stand for the outlet and inlet temperatures of the exchanger, and T0 is the ambient temperature.

• (Tho(i + 1) — T0(i + 1))2 .• Spanel • P

Подпись: Esolar (i + 1) Подпись: Esun(i + 1) •P -K1 • (Tho(i + 1) - T0(i + V) -K2

Using the panel temperature, the control system can decide on the pump operation for the next time step, and finally we can compute the energy input of the solar loop if the pump is on.

(10)

Heat exchange with n+1 and n-1 elements

image222 image223 image224

We consider that only conduction plays an important role in the heat transfer between neighbor elements. Energies transferred to the inferior element and from the superior element within a time step (p) are:

3.1 Energy input from the electrical heater

The auxiliary electrical heater has a 2kW power. Experiments showed that during heating water in the tank is mixed up in the entire volume just above the heater. We consider in this model that all elements are brought at the same temperature after one time step of electrical heating. Thus, the set-up temperature is reached by all elements above the heater in the same time, after one time step or more. If during a time step of heating the hot water temperature might rise above the set-up temperature, the electrical heater will stop even if the time step is not over.

Heat lost to environment

The total thermal resistance of the vertical wall of the tank is:

image216

(1)

 

image217

Подпись: is obtained from the Nusselt number:The convection coefficient in air hair

Nuair = 0 if Ra < 104

Nuair = 0.59 ■ Rar0f if 104 < Ra < 109

Nuair = 0.13 ■ Ra033 elsewhere

The convection coefficient in water hwater is obtained from the next relationship:

image219 image220

г -|2

Supplementary losses appear for the first and the last element, at the top and the bottom of the tank. Different relationships are employed for these two horizontal surfaces in contact with ambient air depending on the temperature gradient. If heat is recuperated from the environment next correlations are computed:

Nuair = 0 if Ra < 104

Nuair = 0.54 • Ra0 25 if 104 < Ra < 109 (4)

Nuair = 0.54 • Ra0 33 elsewhere

And, if heat is lost to the environment, the correlations are:

Подпись: (5)Nuair = 0if Ra < 104 Nuair = 0.27 • Ra0 25 elsewhere

System description. Solar collectors

Two solar glazed panels heat up the heat carrier fluid of the solar loop. The absorbing total surface of the two collectors is 4 m[10] [11]. They are facing south (azimuth angle 0°) and have 45° with the ground. Neither roof integration nor shadows are considered. Collector parameters are: optical coefficient B = 0.81, firs order thermal coefficient k1 = 3.61W/m2K and second order thermal coefficient k2 = 0.0045 W/m2K2 . Stagnation temperature is 215°C’.

2.2 Storage tank

A 300 liters tank is used to storage the solar energy (figure 1). At the bottom of the storage tank an integrated heat exchanger supplies solar energy recovered by the panels. This exchanger is installed inside a stratifier high as 90% of the tank. The tank has also an auxiliary electric heater allowed to run between 22.00 and 6.00 if the upper volume didn’t reached the set-up temperature2. The upper volume heated by the auxiliary heater is 100 liters. No stratification is possible in the volume above the heater when in operation [1].

Tap hot water is delivered at 50°C by a thermostatic mixing valve. This one add cold water to the hot water from the tank.

ixer

 

S teiage tank

 

image212

E iectric heater

 

C ontrol device

 

C oU w ater

 

image213

image214

Fig. 1. Layout of a domestic hot water solar heaters (HWSH).

2.3 Control device

A classic control device is an electronic box able to take two decisions:

• To run or stop the pump of the solar loop depending on the temperatures registered in the collector and at the bottom of the tank;

The new control device is able to decide also the set-up temperature of the auxiliary heater depending on the needs and on the weather forecast [2, 3].

3. System model

Presented model is a finite difference one with nodal discretisation, and is based on the mass and energy balance. The simulations are carried on by dividing the tank in many isothermal elements or slices, with equal volumes. For each element we take into account the next heat transfers and energy interactions (figure 2):

• Heat lost to environment,

• Heat exchange with n-1 element,

• Heat exchange with n+1 element,

• Energy input from the electrical heater,

• Energy input from the solar exchanger,

• Energy input from the n+1 element when hot water is poured,

• Energy output to n-1 element when hot water is poured.

image215

Control of a domestic hot water solar heater with weather forecast

Mihai Radulescu1* and Aude Lepeltier2

1 EDF R&D Site des Renardieres, Dept EnerBat, Avenue des Renardieres — Ecuelles, 77818 Moret sur Loing
2 Ecole des Mines de Douai, 941 rue Charles Bourseul, BP 10838, 59508 Douai Cedex
* Corresponding author, mihai. radulescu@edf. fr

Abstract

This paper presents the efficiency improvement and electricity economy obtained by introducing an advanced control of a hot water solar heater (HWSH) using a weather forecast. In order to quantify the energy gain of the advanced control a numerical model was built for the storage tank. This model takes into account all inlet and outlet energy fluxes and it’s able to determine the temperature inside the tank at any moment and height. The advance control is acting on the set-up temperature of the auxiliary electric heater in order to prepare a reduced quantity of hot water during night and to allow to the solar loop to recover as much as solar energy possible. The comparison between a standard system and a HWSH with weather forecast shows that a 7% annual electrical economy is found and a 3% efficiency improvement.

Keywords: solar thermal, hot water, control system, weather forecast.

1. Introduction

Electricity production in France is mostly nuclear with 88% completed by small parts of natural gas 4% and hydraulic 8%. The most important electricity producer is by far Electricite de France (EDF) with a total annual capacity of about 650 TWh, and just 490 TWh in France.

Face to global warming and approaching oil peak, research efforts turned toward renewable energy sources. Theses technologies can produce “clean” energy without or with low carbon dioxide emissions but remain still expensive and less widespread. One technology, with unlimited resources and a good potential to overcome the actual setbacks, is the solar thermal. Probably the main disadvantages leading to low solar energy use are the intermittent availability and the low reliability of the energy source. Transforming the solar energy in heat seams so simple, but to recover this heat and to use it efficiently becomes more complicated. Heat has to be carried by a fluid (air or water) to be used immediately or to be stored.

This paper focuses on domestic hot water solar heaters (HWSH) with a storage tank and forced circulation of the heat carrier fluid. Such a system is coupled with a meteorological device (not described) to provide the weather forecast. The energy economy and comfort gain are established with respect to a classical HWSH. The description of the system will be followed by a short explanation of the model. The results are divided in two parts: with and without the weather forecast.

Boundary conditions

image211

The simulation study is based on a single family house with a living area of 128 m2 located in Wurzburg, Germany. The roof area where the collectors are mounted is facing south with an inclination of 45o. The space heating demand of the building conforms to the current legal energy saving regulations (EnEV) and amounts to 71 kWh/ (m2 a) respectively 9090 kWh/a. The heating control is automatically adjusted to the outside temperature with a maximum flow/return temperature of 35/25°C. The heat demand for hot water amounts to 2945 kWh/a for a daily use of 200 litres at 45°C. The total heat demand (thermal requirement) for hot water preparation and space heating amounts to 12680 kWh/a, assuming heat losses of a conventional hot water store of 645 kWh/a. The flow temperatures for the brine were calculated based on measurements taken at a heat pump system using a borehole heat exchanger and range from 7°C in February to 20°C in July. The heat pump used in system A shows a COP (Coefficient of Performance) of 4,3 according to EN 14511 at 5K temperature difference. In system B a store-integrated heat pump is used which has a different thermal behaviour due to its positioning. This heat pump features a COP as per EN 14511 of 4,1 at 5 K temperature difference. The same flat plate collector with a total aperture area of 12 m2 and performance parameters of a “good” flat plate collector was used for both systems.

3. Results

Table 1 shows the most important results of the annual simulations carried out for the two systems A and B. In addition the thermal behaviour of a pure heat pump system without solar collectors (system C) was simulated for comparison. This system uses a similar combistore as system A, however with a smaller volume of only 400 litres. For all systems the electric energy consumption for the hydraulic pumps was not taken into consideration for reasons of simplification.

Table 1: Results of annual system simulation

System

A

B

C

„Usable hot water volume“

[litres]

240

260

240

Collector gain: total/

[kWh]/

5068/

4031/

-/

thereof delivered in combistore

[kWh]

3416

3685

Heat losses of combistore

[kWh]

1135

1007

898

Delivered heat by heat pump

[kWh]

9814

9319

12920

Seasonal performance factor [-]

4,4

3,8

4,0

electric energy consumption of the heat pump

[kWh]

2229

2459

3257

Table 1 shows that system B requires the lowest amount of heat delivered by the heat pump in order to cover the entire heat demand for hot water preparing and space heating. Furthermore, this system provides the highest “usable hot water volume” and therefore offers the greatest hot water comfort. The positive thermal behaviour of system B is due to the efficient technology for hot water preparation (external heat exchanger combined with a controlled pump) and to the fact that at this system the highest solar energy gains are delivered to the combistore (3685 kWh). Moreover, the combistore of system B shows less heat loss than the combistore of system A. In system A the heat delivered from the collectors to the combistore is lower than in system B due to the control strategy of the collector loop pump: Within regular waiting periods during charging the buffer store it is checked if it is possible to charge the combistore. Nevertheless, system A requires the least electric energy consumption of the heat pump. That is because system A shows the highest COP due to the high temperature level in the buffer store which can occasionally be used as heat source for the heat pump. In comparison to system C (without solar thermal contribution) system A requires approx. 1000 kWh less electricity in order to operate the heat pump. The annual COP increases from 4,0 to 4,4 due to the solar thermal system. As can be expected, system A shows a high collector energy gain due to the additional buffer store.

4. Conclusions

The investigations have shown that the combination of a solar combisystem with a heat pump is a promising approach for saving primary energy. Due to the use of an additional buffer store in system A the electric energy consumption is less than in system B despite the higher delivered heat of the heat pump. This is due to the higher seasonal performance factor of the heat pump in system A. An

additional saving potential in system B can be utilised if the solar circuit is coupled with the brine circuit by a heat exchanger in order to preheat the return flow of the brine.

In order to compare different combinations of solar thermal systems and heat pumps in an objective way it is essential that standardised test — and evaluation procedures become available.

References:

[1] Kuhl, L., Wendker, K., Fisch, N.: Praxistest von solarunterstutzten Warmepumpen-Heizsystemen; Tagungsband zum 17 Symposium Thermische Solarenergie, Otti-Technologie-Kolleg, Regensburg, Mai 2007; (2) ENV 12977-2: 2001: Thermal solar systems and components — Custom built systems — Part 2: Test Methods, ISBN 0-580-37754-7

[2] EN 14511-3: Air conditioners, liquid chilling packages and heat pumps with electrically driven compressors for space heating and cooling — Part 3: Test methods

Twin-Store-System with heat pump (system A)

Подпись: Fig. 1: Twin-Store-System with heat pump (system A)

In the Twin-Storage-System (see figure 1) a combistore (nominal volume 600 l) is used for hot water preparation (by means of a stainless steel tube) and for space heating. It is charged by the collector via an immersed heat exchanger located in the lower part. The auxiliary part of the combistore is charged by the heat pump. Another pressureless buffer store (nominal volume 800 l) serves in addition to the borehole heat exchanger as a heat source for the heat pump. This store is charged by the solar collector only.

2.1. System with integrated heat pump (system B)

In this system (see figure 2) the heat pump exclusively uses a borehole heat exchanger as heat source. The heat pump is fitted at the combistore (nominal volume 750 l) and the condenser is located in the auxiliary part of the combistore. The return flow of the space heating loop is connected to the combistore using a special device for stratified discharging of the store. The heat gained by the collector is added to the combistore via an internal heat exchanger which is also equipped with a special stratification device. An external heat exchanger is used for hot water preparation.

Solar thermal systems combined with heat pumps — investigation of different combisystem concepts

S. Bachmann, H. Drtick, H. Mtiller-Steinhagen

University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW) Pfaffenwaldring 6, 70550 Stuttgart, Germany Tel.: +49 711 / 685-63553, Fax: +49 711 / 685-63503

Corresponding Author, email: bachmann@itw. uni-stuttgart. de

Abstract

Two solar combisystem concepts have been investigated where a compression heat pump is used as the only auxiliary heater. The investigations have shown that the combination of a solar combisystem with a heat pump can result in a promising approach for saving primary energy. This especially holds true compared with a monovalent heat pump system (without a solar thermal system). However, the investigated systems show a great difference concerning the electric energy consumption of the heat pump. Especially systems where the brine is solar preheated can offer a great reduction of electric energy consumption for the heat pump.

Keywords: compression heat pump, solar combisystem, seasonal performance factor, component testing

1. Introduction

Heat pumps are presently enjoying a wide popularity in Europe. In 2007, the market for heat pumps showed a growth rate of about 100% in Germany. In recent years heat pumps were predominantly used as monovalent systems or in combination with a fossil fuel fired auxiliary heating system. At present, however, a number of new heating systems are being launched on the market where the heat pump is used as the only auxiliary heater for a solar combisystem. In addition to biomass heating systems, heat pumps offer the possibility of supporting the solar thermal system by, at least partly, renewable energy. Depending on the individual system design, there is a particular charm in combining heat pumps with solar thermal systems because this combination can provide an additional heat sink with a low temperature level so that the solar collector energy yield can be increased. For the heat pump on the other hand this approach offers a heat source with a relatively high temperature level which is beneficial for the coefficient of performance of the heat pump.

At the Research and Testing Centre for Thermal Solar Systems (TZS) in the Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, two solar combisystems combined with a compression heat pump were investigated based on the component testing approach as described in the European Standard ENV 12977-2. One system consists of a compression heat pump where the condenser is located in the auxiliary section of the combistore. The other system uses an external compression heat pump in combination with an additional water store as heat source for the heat pump, which is exclusively charged by the solar collectors.

This paper presents the different system concepts. The results of the investigations such as the electric energy consumption of the heat pumps required to cover the total heat demand of the building, will be presented and discussed.

2. The investigated systems

Year round performance

Fig.6 shows the year-round performance of the solar DHW system. In Fig.6 the collector efficiency, the DHW heat load, the solar contribution, the CO2 emission and energy cost are compared. The performance in Fig.6 is expresses for a housing unit. The CO2 emission was calculated from the gas calorific value of 45MJ/m3 and the CO2 emission coefficient of 2.21kg/m3. The energy cost was calculated using the fee structure of the city gas company in Tokyo. The average unit price of the gas is 149JPY/m3. The collector efficiency for the collector area of 20m2 with Case A DHW supply is from 53.0% to 63.4%. It tends to increase according to the storage tank volume is large. The collector efficiency decreases by 3.6%-7.2%, when the collector area increases by 10m2. The considerable difference in the collector efficiency is found when increasing from 0.5m3 to 1.5m3 of the storage tank volume, however there is only few difference when the storage tank volume is more than 2.0m3. The collector efficiency for the collector area of 30m2 and the storage tank volume of 1.0m3 is 53.0% with Case A DHW supply, 49.5% with Case B DHW supply, and 41.6% with Case C DHW supply, respectively. The CO2 emission for collector area of 30m2 and the

image207

Fig.6 Year-round performance from the simulation results expressed for a housing unit.

 

storage tank volume of 1.0m3 is 650kg-CO2/year with Case A DHW supply, 300kg-CO2/year with

 

Case B DHW supply, and 67kg-CO2/year with Case C DHW supply, respectively, for a housing unit.

Relationships of the collector area and the storage tank volume for the solar contribution is shown in Fig.7. The solar contribution increase by 4%-26% when collector area increases by 10m2 in all simulation cases. In addition, the solar contribution increases with larger collector area and larger storage tank volume. The solar contribution only increase

 

by 2.2% when the storage tank volume is increased from 1.0m3 to 1.5m3 in case of the collector area of 20m2 and Case A DHW supply. However, the solar contribution only increase by 6.7% when the storage tank volume is increased from 1.0m3 to 1.5m3 in case of the collector area of 50m2 and Case A DHW supply.

image208
image209

The comparison of the saved cost of the city gas for a housing unit was shown in Fig.8. The dark bars in Fig.8 show the suitable combinations of the collector area and the storage tank volume judged from the solar contribution. The saved energy cost for the collector area of 30m2 and the storage tank volume of 1.0m3 with Case A DHW supply is estimated to be 19,700JPY/year for a housing unit. The comparison of the installation cost for 10 housing unit is shown in Fig.9. The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 with Case A DHW supply should be suppressed 2.0 million JPY when the pay back period of 10 years is assumed. The installation cost should be suppressed 3.0 million JPY when the pay back period of 15 years is assumed. The saved energy cost for the collector area of 20m2 and the storage tank volume of 0.5m3 with Case A DHW is estimated to be 80,000JPY/year, and the installation costs are 8.0 million JPY and 1.2 million JPY for pay back period of 10 and 15 years, respectively.

Fig.9 The installation cost for 10 housing units assuming the pay back periods of 10 and 15 years.

6. Conclusion

The central type of solar DHW supply system for 10 housing units was simulated to examine the relationships of the collector area and the storage tank volume with considering the DHW supply rate. The simulation results showed the followings.

1) The collector efficiency for the collector area of 30m2 varies from 38% to 59% depending on the storage tank volume and the DHW supply rate. The collector efficiency decrease by 3.6%-7.2%, when the collector area increases by 10m2.

2) The suitable storage tank volume is from 1.0m3 to 1.5m3, since the collector efficiency decreases extremely when the storage tank volume is 0.5m3. The efficiency for 2.0m3 is almost same as the case for 1.5m3.

3) The solar contribution for the collector area of 30m2 and the storage tank volume of 1.0m3 is 37.1% with Case A DHW supply, 52.5% with Case B DHW supply, and 74.6% with Case C DHW supply, respectively.

4) The boiler load for the collector area of 30m2 and the storage tank volume of 1.0m3 is 105.9GJ/year with Case A DHW supply, 48.6GJ/year with Case B DHW supply, and 10.8GJ/year with Case C DHW supply, respectively.

5) The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 should be suppressed 2.0 million JPY with Case A DHW supply, 1.7 million JPY with Case B DHW supply and 1.0 million JPY with Case C when the pay back period of 10 years. The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 should be suppressed 3.0 million JPY with Case A DHW supply, 2.5 million JPY with Case B DHW supply and 1.5 million JPY with Case C when the pay back period of 15 years.

References

[1] T. Kusunoki and M. Udagawa, Planning of Solar Collector Arrangement for Solar DHW Heating Apartment House, Proceedings of JSES/JWEA Joint Conference 2006, pp. 125-128. (In Japanese)

[2] H. Roh and M. Udagawa, Study on Standardization of Solar DHW Heating System for Apartment Houses, Proceedings of JSES/JWEA Joint Conference 2005, pp.79-82. (In Japanese)

[3] M. Udagawa, H. Roh and M. Satoh, Design of Solar DHW System for Apartment Houses, Proceedings of ISES Solar World Congress 2005

[4] M. Udagawa, and M. Satoh, Energy Simulation of Residential Houses Using EESLISM, Proceedings of Building Simulation ‘99, pp.91-98. (In Japanese)

[5] Architectural Institute of Japan, Expended AMeDAS Weather Data, 2005. (In Japanese)

[6] S. Kaneko, M. Udagawa and T. Kusunoki, Design of Solar DHW Heating System for Small Apartment House, Proceedings of JSES/JWEA Joint Conference 2007, pp.213-216. (In Japanese)