Category Archives: EuroSun2008-7

Task B: Performance evaluation

This task is divided in work packages born from the difficulty in comparison of the newly developed machines. Not all systems existing in the market are able to be tested with the published norms created some years ago for great capacity sorption chillers producing 7 °C water with the classical temperature levels in the generator and condenser, not only because of the sizes but also for the different technologies that are going to be taking into account, absorption cycles, adsorption and ejectors.

Nominal Coefficients of Performance (COP’s) of this new machines are being measured at different temperature levels depending on their final use. This means, that results are not directly comparable among them, due to the diversity of nature of values. Therefore, a new way to compare them based on the apparatus and the complete system performance is needed.

As the COP’s of the machines themselves are being calculated, also the efficiencies of the facilities should be determined due to the number of pumps and different equipments needed.

Furthermore, a parametrical study with the different energy supplies that can be used must be done, which may range from solar thermal plants to district heating and waste heat sources etc.

District heating systems can be connected to TDHP (i) in winter time to heat with an incremented performance and (ii) in summer time to cool — keeping more stable the heat demands from the point of view of the production. As can be seen in the figure3, presented in the 2nd workshop meeting (Zurich, 20th May,08) during the months of July and August, the heat demands decreases to a 15% of the nominal one, creating a yearly overall inefficiency in the design of the system and the distribution of that heat. With an existing demand to produce cold in these summer months, the differences between summer and winter will decrease.

image517

Fig. 3. Demand profiles of four installed District Heating Systems in Germany.

Collaborating with the Task38, the use of solar energy as the energy vector which drives, or helps to drive, the TDHP is being studied. In this case, solar radiation will be used during the winter as low level input for the heat pump, highly increasing the efficiency of the collectors, although the amount of radiation in certain climates will not be so much. Previous calculations in [6] have been done in order to explain this.

Depending on the final distribution systems (i. e. Fan coils, radiant ceilings and floors, etc) will be need as an output of this task a new methodology to label and recommendations for test procedures where it will be taking into account the different temperatures for heating and cooling, COP’s, energy consumptions, CO2 production, exergies, etc.

Most common housing typologies

The most common recent-built-housing in southern Spain could be classified into three typologies:

• Multi-family building with six 60-square-mettered apartments

• Single-family 120-square-mettered terraced house

• Single-family 400-square-mettered detached house

There are many differences between the three housing typologies listed. From the point of view of cooling, the most remarkable differences are those related to the simultaneousness factor and the level of comfort required in each one.

1.1. Traditional cooling technologies

Up to now, the domestic systems most frequently installed in Spain have been mono or multi-split mechanical vapour compression systems, due to its modularity, price and technical development. Other solutions, such as centralized chillers, have been used for large-surfaced housings, but are not very common for domestic purposes.

Design of a polymer reflector carrier based parabolic trough collector

1.1. Design specifications

When the authors set forth to develop a parabolic trough collector system especially targeted at commercial buildings the following design criteria were seen as important:

• Aesthetic appeal with low height

• Modular system allowing also small fields (say as small as 50m2) to be economically viable

• Easy to install; not requiring special tooling or lifting equipment

• Accommodate alternative layouts and orientations on complex roof geometries

EUROSUN 2008

image0261st International Congress on Heating, Cooling, and Buildings 7th to 10th October, Lisbon — Portugal *

• Light weight for roof mounting with few foundation points

• Accuracy and stiffness to ensure high performance

• Cost competitive on a $/kWh level

• All components shipped in 20 ft ISO sea containers

Development of a solar desiccant cooling system for small office buildings

H. Roh1*, K. Suzuki1 and M. Udagawa2

1 OM Environmental Planning, 4601 Murakushi-Cho, Hamamatsu-City, Shizuoka-Prefecture, 431-1207,

Japan

2 Kogakuin University, 1-24-2 Nishi-Shinjuku, Shinjuku-Ku, Tokyo, 163-8677, Japan

* roh@omsolar. jp
Abstract

This paper describes the development and the field test of a solar desiccant cooling system for small office building. The main purpose of this system is the process of the fresh air heat load and the getting cool to dehumidify the fresh air. This system is composed of two air handing units with two desiccant modules and is operated in regeneration process and in dehumidification process at the same time while the operation mode is switched. Solar heat is used for the regeneration of desiccant module and well water is used for cooling the air passed through the desiccant module. From the monitoring results, the fresh air heat load of 73% was reduced by the effect of the solar desiccant cooling system. From the field test results, it was found that the reduced daily electric power consumption of the electric heat pump air conditioner (EHP) was 36% for the operation of the solar desiccant cooling systems compared with the operation stop. In winter, these solar desiccant cooling systems are used for the solar space heating systems. From the field test results in winter, the reduced daily electric power consumption of the EHP was 40% for the operation of the solar space heating systems compared with the operation stop.

Keywords: solar thermal, desiccant, cooling, well water

1. Introduction

The common air-based solar thermal system is useful for space and DHW heating, whereas the collected solar thermal energy is almost exhausted to outdoor except heating DHW in summer. For using the solar thermal energy available throughout a year, the solar desiccant cooling system has been developed for the pre-cooling system of fresh air to dehumidify fresh air with ventilation in summer. In addition, this system is used for solar space heating system to heat fresh air with ventilation in winter. This system was installed at the field test office building in the central area of Japan. The mechanism of the solar desiccant cooling system and the results from the field test is introduced.

Regenerative Evaporative Cooler

The schematic of the regenerative evaporative cooler fabricated for the application to the desiccant cooling system is shown in Fig. 3. The regenerative evaporative cooler is comprised of multiple pairs of the dry and wet channels as shown in Fig. 3. A representative single pair of the channels is displayed in Fig. 3(a). The two channels are separated by a thin flat plate and metal fins are inserted into both the channels to extend the contact surfaces improving the compactness of the cooler. A guide channel is attached at the bottom end of the wet channel to prevent mixing between the process air inflow and the extraction air from the wet channel.

The regenerative evaporative cooler was built by compiling multiple of the single pairs. The detailed geometric configurations such as the frontal area, the number of pairs, the fin height, the fin pitch, etc. were determined through numerical simulations on the heat and mass transfer in the channels to meet the cooling requirement.

Portugal *

Подпись: supply airПодпись:image259dry channel

y V*

wet channel

extraction air

guide channel

(a) single pair of the channel (b) assembly of multiple pairs

Fig. 3. Fabrication of the regenerative evaporative cooler.

The regenerative evaporative cooler was tested in a climate chamber for the cooling performance evaluation at various inlet temperature and humidify conditions. The flow rates of the two air streams, i. e., the process air and the extraction air, were measured at the outlets of each channel. The temperature and the humidity were measured at both the inlet and outlet. The energy balance was examined by comparing the enthalpy changes in the two air streams and was found within 10% error for every test case.

The representative test results are displayed in Fig. 4. The tests were carried out at the flow rate of the inlet air of 20 CMM and the extraction ratio of 0.3. When the inlet air is at 32oC and 50%RH, the outlet temperature is found about 22oC showing the cooling effect of 10oC. This outlet temperature is even lower by 2oC than the inlet wet-bulb temperature which is the lowest temperature obtainable in a direct evaporative cooler. Furthermore, comparing with the adiabatic process in the direct evaporative cooler, the cooling in the regenerative evaporative cooler is substantial, since the cooling is obtained without an increase in the humidity. The outlet temperature is also shown to decrease as the inlet humidity decreases. This is because the water evaporation becomes more active to increase the cooling effect as the air becomes dryer.

2.

Подпись: (b) effect of inlet temperature
image261

Desiccant Rotor

Fig. 4. Cooling performance of the regenerative evaporative cooler.

A desiccant rotor was fabricated using the polymeric desiccant newly developed in Korea Institute of Science and Technology (KIST)[9]. This polymeric desiccant is developed by ion modification of super absorbent polymer (SAP) and is named as super desiccant polymer (SDP) for its superior moisture sorption capacity 4~5 times larger than those of conventional silica gels. It is also known that SDP can be regenerated even at the relatively low temperature of 60 ~ 80oC.

To fabricate the desiccant rotor, firstly the SDP was prepared by ion modification of SAP and laminated by coating the SDP on a 0.1 mm thick polyethylene sheet. Then the sheet was corrugated and rolled up into a rotor. Figure 5 shows the completed desiccant rotor framed into a cassette.

The diameter, the depth, the dimensions of the corrugated channel, etc. were pre-determined from numerical simulations on the heat and mass transfer in the desiccant rotor.

The dehumidification performance was also tested in a climate chamber. The flow rates of the two air streams, i. e., the process air and the regeneration air, were measured at the inlets of each channel. The temperature and the humidity were measured at both the inlets and outlets of the two air streams. The energy balance was examined by comparing the enthalpy changes in the two air streams and was found within 10% error for every test case. The moisture balance between the two air streams was also found within 10% error for every case.

Figure 6 shows the dew-point temperature depression and the temperature increase of the

Подпись: (b) process outlet temperature

image263 image264

(a) dewpoint depression

Fig. 6. Effect of the rotation period of the desiccant rotor.

process air through the desiccant rotor. The relevant tests were carried out at the process air inlet temperature of 32oC, the regeneration air inlet temperature of 53oC, and the inlet dew-point temperature of both the process air and the regeneration air of 18.5oC. As shown in Fig. 6(a), the dew-point temperature depression is maximized at a certain rotation period of the desiccant rotor, which implies the dehumidification performance is optimized at the rotation period. When the rotation period is excessively short, the moisture sorption of the desiccant is not effective because there is not enough time for the desiccant to be cooled from the regeneration temperature down near to the temperature of the process air which is necessary for the desiccant to absorb moisture from the process air. In the meanwhile, when the rotation period is excessively long, the moisture sorption is not effective either because the desiccant becomes saturated with moisture resulting in a decrease in the moisture sorption capability. In these reasons, there comes to exist an optimum rotation period of the desiccant rotor. In the desiccant rotor developed in this study, the optimum rotation period is found about 500 seconds at the regeneration temperature of 53oC. It was found from further experiments that the optimum rotation period tends to decrease as the regeneration temperature increases. Meanwhile, the outlet temperature of the process air decreases monotonically as the rotation period increases.

Active Solar-driven Cooling Systems

The central part of this building’s cooling system is a solar powered absorption cooling system made up of four 10kW state-of-the-art absorption heat pumps, driven by 90 high performance flat- plate solar collectors able to constantly deliver fluid at up to 95°C, the maximum temperature for the absorption heat pump to work properly. In summer this system is a serious alternative to conventional chillers or heat pumps for providing cold water to the cooling coils as it requires almost no additional energy (apart from the solar energy captured by the solar collectors) to cool the building. The absorption heat pump used at the Almeria building uses a patented technology that allows energy to be stored and instantly delivered later on. This way the system can store energy all along the sunny hours of the day and keep on cooling down the building even after sunset if necessary. It has to be mentioned that the system is designed so that it can store and deliver at the same time. Also, the technology employed allows the input fluid temperature to vary, as long as the temperature difference between the source and the heat sink (in our case a cooling tower for practical reasons) is above 50°C. When considering using solar thermal collectors as a heat source, this is an advantage over conventional absorption chillers that need the input fluid to remain at a constant temperature. The cooling tower can actually be controlled so that the absorption system keeps working even when the solar collectors’ output temperature falls down. On the other side, the system can also be set to take higher temperatures of up to 120°C for some minutes so that, in the end, this system makes it possible to take advantage of all of the available solar energy to produce cooling energy for the building.

image391
image392

Fig. 1. Array of solar collectors on the roof (left) and Absorption heat pumps (right).

Standard desiccant systems

1.1. The case study

The main purpose of this study is the comparison between the performance of different concepts for DEC systems in Mediterranean climates. This comparison will be performed on a first and second law perspective; the analysis will be applied to the following systems: •

thermal load QL = 2.5 kW and a sensible thermal load QS = 7.5 kW. In Table 1 the design indoor and outdoor conditions are reported, as well as the inlet air conditions, determined by assuming an inlet air flow

ma = 1 kg/s (Va ~ 3200 m3/h); the regeneration air flow is equal to the process air. The design outdoor conditions are typical of a hot and humid climate, such as the one occurring in the countries of the Mediterranean area.

Table 1. Design conditions for the case study.

Indoor

conditions (A)

Outdoor conditions (E)

Inlet

conditions (I)

Dry bulb temperature

26 (°C)

35 (°C)

18.5 (°C)

Relative humidity

50 %

60 %

72 %

Vapour content

10.5 (g/kg)

21.4 (g/kg)

9.5 (g/kg)

Enthalpy

52.8 (kJ/kg)

89.9 (kJ/kg)

42.6 (kJ/kg)

Thermal stability and fluid compatibility with materials

Thermal stability and fluid compatibility with materials as well as with lubricants in contact is a critical step in the design of an ORC plant. The working fluid must have a high thermal stability to provide the desired lifetime and a cost-effective plant. Care should be taken to make sure that the combination fluid/lubricant/material can assure a long lifetime period of the plant. Among the materials used are copper, steel and stainless steel just to name a few. The lubricant can be miscible or immiscible with cycle fluid, but for minimum system complexity miscible oil is desirable. Chemical decomposition of the fluid not only reduces the plant efficiency and makes the replacement of the fluid necessary but can produce non­condensable gases which have corrosive effects on the materials of the system. In order to study the fluids decomposition, two methods are available: the dynamic loop tests and the static capsule tests. The first method is the best but time-consuming and expensive. In general, to safeguard against plant damage from fluid decomposition, a combination of static capsule tests, large safety margins and field monitoring is usually employed. Wali [8], Calderazzi and Colonna di Paliano [9], Angelino and Invernizzi [10] are some authors who investigated the thermal stability of fluids. In Table 3 are displayed the maximum stability temperature of some fluids.

Table 3: Maximum stability temperature (MST) of some fluids

Fluid

MST (°C)

Material

References

R227ea

425

Stainless steel (AISI 316)

[10]

R23

400

Stainless steel (AISI 316)

[10]

R236fa

400

Stainless steel (AISI 316)

[10]

R143a

350

Stainless steel (AISI 316)

[10]

R245fa

300

Stainless steel (AISI 316)

[10]

R134a

368

Stainless steel

[9]

R141b

90

Stainless steel

[9]

R13I1

102

Stainless steel

[9]

R7146

204

Stainless steel

[9]

R125

396

Stainless steel

[9]

Methanol

175-230

[2]

Toluene

400-425

[2]

R113

175-230

[2]

FURTHER R&D ON ZEOLITE HEAT PUMPS

The silica gel machine is working at very low driving temperatures down to 65°C, but may not be used, when low water temperatures and high re-cooling temperatures are necessary. In order to achieve this temperature region SorTech AG is currently developing a second prototype chiller based on the same construction principles but using zeolite coated adsorbers.

A new type of coating has been developed and filed for patent application by SorTech AG, which is based on a direct crystallisation of the zeolite on the surface of the heat exchanger (Fig. 5). Compared to the epoxy resin coating which is used in ACS 08 this new technology has the following advantages:

High mechanical stability of the coated layer due to strong binding forces at the interface zeolite — metal.

Fast adsorption kinetics, because of good heat and mass transport resulting from very compact layers, which contain only the active material.

Подпись: Fig. 5. REM picture of the cross-section of an aluminium fin coated with zeolite by direct crystallisation. The overall thickness of the fin is about 500 microns.

Reduction of material synthesis and coating to a one step process.

The feasibility of the direct crystallisation method has been shown by SorTech AG on small samples of aluminium fin material and on tube and fin heat exchangers. Currently we are working on the up-scaling of the process in a pilote plant as a first step for a later industrialisation.

Due to the reaction conditions of the zeolite (200 °C at 20 bars) the chemical engineering is a rather challenging task und is currently being performed.

6. CONCLUSIONS

Small compact adsorption chillers seem to be an attractive option for thermal driven air conditioning. The results of the field test of prototypes carried out in 2007 were encouraging. Therefore since March 2008 SorTech AG has introduced a small adsorption chiller ACS 08 with a chilled water capacity of 7.5 kW and since June 2008 a larger system ACS 15 with 15 kW into the market. Currently, we are producing our products in a small series and the market response is very positive.

7. ACKNOWLEDGEMENT

The financial support from the German Federal Ministry of Economics and Technology (BMWi) is greatly acknowledged.

Performance assessment via exergy method

Second law analysis is a useful tool for the identification of irreversibilities and, in result, improvement potentials of thermal energy systems. Exergy enables the quantification of a system’s potential to perform work when brought in equilibrium with the environment. As opposed to energy, exergy can be destroyed. The exergy analysis of a thermal system therefore helps identifying the loss and destruction of available energy, identifying where the “true” potential to perform work is not exploited.

1.1 Exergy equations for the evaluation of HVAC cycles

The application of the exergy method to moist air processes such as air-conditioning, drying and wet cooling tower processes was first described by Szargut [4]. A pathbreaking publication for the application in HVAC was later written by Wepfer [5]. Processes of evaporative cooling [6,7] and rotary type desiccant systems [8,9] were later both covered. Equation 3 is the generally applied moist air specific exergy equation written on a per mass of dry air basis [4,5]:

image085

[J/kg] (equation 3)

where the three terms give the thermal, mechanical and chemical exergy of moist air. An important issue for the application of exergy to moist air is the selection of the dead state. In the literature, both selecting either ambient conditions or saturated air at ambient temperature is discussed. The latter approach discussed by Chengqin [7] is followed in this paper, considering that unsaturated air still has a potential to perform work as it undergoes a temperature drop when humidified. Ideally, a Carnot engine could then be driven between the ambient air and the humidified air. According to equation 4, the moist air specific exergy is falling monotonously with rising moist air humidity ratio.

The exergy of water used for evaporation is generally described by equation 5 and is derived from analyzing a process where water is condensed from ambient air [5]:

= h(T) — h(To) — To[(T) — s(To)] ■+ [p — PSat (T)V(T) — RvT0 In Po [J/kg] (equation 5)

When choosing ambient temperature and humidity ratio as reference conditions, the last term is dominating. However, when choosing saturated air at ambient temperature as the dead state as followed here the last term drops out. For evaporative cooling schemes, the latter approach was found to give more reasonable results by the authors as the evaporation of water would otherwise be insensibly penalized.