Energy analysis

The results obtained for the different simulations are summarised in the tables 1 and 2. They show the required solar collector surface to achieve the objective of 700 MWh/year of chilled water and

different energy performance parameters for the different thermal chiller and solar thermal technologies.

As expected, both tables show that solar collectors having higher efficiency parameters give a higher heating and cooling production per area unit. Also it could be observed that because of the lower operation of the solar adsorption systems, these have higher solar gain coefficients than the absorption ones. Alternatively, the specific chilled energy coefficients depends not only of the collector technology but also of the thermal chiller technology, being the best options the ab/adsorption systems with ETC-CPC collector with a maximum performance of 606 kWh/m2y for the combination with the BDH-65 chiller. The adsorption systems specific chilled energy coefficients only exceed the ones of the absorption systems when FPC collectors are used.

Comparing the Broad solar cooling systems themselves, the BDH-65 systems give a better performance than the BDH-50 ones, especially for ETC and FPC collectors. That fact leads to lower solar collector surface requirement to achieve the 700 MWh of chilled energy. The reason of that behaviour is the better annual average performance of the thermal collectors in BDH-65 systems. Looking at only to the adsorption results of table 2, the MYCOM ADR-60 SYSTEMS present better performance than the ADR-80. In that case the solar system has more or less the same performance and the explanation of this result is that the ADR-60 systems operate at slight higher temperatures in the generators-receivers, obtaining then slight higher values of their COP.

T able 1. Collector surface and energy performance parameters for different solar collector technologies and

absorption chillers.

Chiller

TRNSYS

model

Collector

type

Gross

Collector Area m2

Solar

gain

MWh

Spec. Solar gain

kWh/m2y

Annual chilled energy

MWh

Spec chilled energy

kWh/m2y

Seasonal

COPchiller

Broad BDH-65 663 kWc

AAt’[2]

Type811

FPC

1888

919

487

700

371

0.76

ETC

1571

919

585

700

446

0.76

ETCCPC

1155

919

796

700

606

0.76

Broad BDH-50 512 kWc

AAt’[2]

Type811

FPC

2079

918

441

700

337

0.76

ETC

1623

918

566

700

431

0.76

ETCCPC

1165

918

788

700

601

0.76

Table 2. Collector surface and energy performance parameters for different solar collector technologies and

adsorption chillers.

Chiller

TRNSYS

model

Collector

type

Gross

Collector

Area

m2

Solar

gain

MWh

Spec. Solar gain

kWh/m2y

Annual chilled energy

MWh

Spec chilled energy

kWh/m2y

Seasonal

COPchiller

2xMycom

ADR-80

560kWc

AAt’[2]

Type811

FPC

1665

1131

679

700

420

0.62

ETC

1637

1131

691

700

428

0.62

ETCCPC

1340

1131

844

700

523

0.62

2xMycom

ADR-60

422kWc

AAt’[2]

Type811

FPC

1631

1120

686

700

429

0.62

ETC

1607

1120

697

700

435

0.62

ETCCPC

1328

1120

844

700

527

0.62

Comparing this analysis to the previous study [3], it could be observed that the MYCOM ADR-80 results are very similar with differences lower than 3.5 %. In the previous study we used some empirical correlations to simulate that chiller. This entire means that the method suggested by Ktihn and al. [3] is valid to model this adsorption chiller. As regards the absorption chiller model, in [3] we used the Thermax LT 21 S with 739 kW with an average COP of 0.64. In the case we are dealing with now we selected the Broad chillers because they present a better COP (0.76). As a consequence, the values of the specific chiller capacity calculated now are between a 4 and 15 % higher. Obviously, the collectors’ surface requirements are also reduced in the same amount.

It should be remarked that, due to the special chilled water demand profile, the solar cooling system should be in operation the whole year. As a result the cooling water temperature is almost 2/3 of the year close to 22 °C. One of the most important consequences of working at lower cooling tower water temperatures is that the temperatures needed in the generator of the chiller could be lowered maintaining the capacity and then increasing the performance of the solar thermal field. In fact, the temperature in the generator can be as low as 60°C for adsorption and 85°C for absorption in the winter period.