Solar cooling using low power sorption refrigeration systems driven by renewable energy sources for sustainable buildings

SERBAN ALEXANDRU, FLOREA CHIRIAC, GABRIEL NASTASE

Building Services Department Transylvania University of Brasov

Turnului Street, No. 5, Brasov ROMANIA

alexandru.serban@unitbv.ro http://www.unitbv.ro Abstract: - The paper presents a cooling system for buildings, consisting of water-silicagel adsorption machines, with a capacity of 5 to 20 kW, which can be powered by renewable energy sources (solar, geothermal) and also with secondary (recoverable) energy sources (hot water, condensate, hot gas etc.), the temperature of these energy sources being higher than 80 oC. Those machines have a low energy efficiency (small COP) but by using renewable/recoverable energy sources they have the advantage of producing cold using a free source and also present a higher reliability as a result of the reduced moving parts meaning also a low noise operation. Those machines uses refrigerants having zero ozone depletion potential and low global warming potential. Because we need cold energy exactly when outside is too hot these machines are ideal for air conditioning in the summer. Key-Words: - building, adsorption, air conditioning, renewable energy 1 Introduction

The idea of solar cooling is not a new one. Passive cooling of buildings dates back to ancient times. Passive cooling is defined as attaining comfort by means of evaporative cooling, thermal inertia of the building, ventilation and shading. However, passive solar cooling alone is not enough to obtain thermal comfort always. Active solar cooling technologies are used to complement it. Active solar cooling systems are driven by cost effective heat sources, mostly with solar energy. 2 Adsorption refrigeration system

Adsorption cooling technology is relatively new, clean and which can use heat sources of low heat potential. An adsorption cooling system is similar to the cooling system with mechanical vapor compression, except that the energy needed by the compression process is provided by heat instead of mechanical work. Unlike the classic mechanical compression cooling, which requires a mechanical compressor, the adsorption cooling system uses a static thermally activated sorbent layer. In this way the compression energy requirement drops to 90% over the energy needed to drive the mechanical compressor. The most common pairs of working substances are water-silica gel, metal chlorides and

composite adsorbents. Continuous adsorption cooling cycle is presented in Figure 1.

Fig. 1. Schematic drawing of two beds adsorption cooling

2.1 Adsorbents and refrigerants

There are three type of absorbents used in adsorption refrigeration systems categorized as physical, chemical and composite adsorbents. The commonly used physical adsorbents used in adsorption refrigeration system are silica gel, activated carbon and zeolite. The most common chemical adsorbent-refrigerant pair is metal chlorides, such as CaCl2 (calcium chloride), SrCl2 (strontium chloride), MgCl2 (magnesium chloride),

Computer Applications in Environmental Sciences and Renewable Energy

ISBN: 978-960-474-370-4 28

BaCl2 (barium chloride), MnCl2 (manganese chloride), CoCl2 (cobalt chloride) and NH3 (ammonia). Ammonia is the most frequent used refrigerant for the industrial applications (food cooling mainly), but also for air conditioning.

Every category have specific advantages or disadvantages, characteristics but this article is focused on physical water-silica gel pair because it has the advantage of exploiting low-temperature heat sources. The silica gel is a porous solid produced from sodium silicate or sulfuric acid and is recommended in adsorption refrigeration because of his great capacity to absorb dry water vapors, around 30-40% along with low regeneration temperatures. Global warming potential of refrigerants as a huge challenge for the future, and for all countries, leads to natural refrigerants as an efficient, cost-effective and safe solution for many applications in refrigeration, air conditioning and heat pumps. For technical and safety reasons they often require different equipment than « chemical » refrigerants (CFCs, HCFCs, HFCs). Even if natural refrigerants still require improvements, research and development in various applications they should be better promoted for new equipment but for the replacement of old equipment too being a cost efficient alternative. In this context it can be said that silica gel is a natural substance,

cheap, environmentally safe, chemically stable, non-corrosive, has a great adsorption capacity, does not age and can be regenerated in the process without loss of adsorption capacity.

To evaluate the performance of an adsorption system and adsorbents two indicators are widely used, namely, COP and SCP (specific cooling power: the ratio of cooling capacity to mass of adsorbent in the adsorbers).

2.2 Mathematical model of the adsorption cooling system

In order to evaluate an adsorption system, namely the coefficient of performance – COP, a mathematical model for the system is needed.

The model is based on the following assumptions: The temperature and pressure distributions in

the heat exchangers are uniform; The refrigerant can be uniformly adsorbed by

the adsorbent; The heat exchangers are well-insulated, and

there is no heat loss to the surroundings. The lumped parameters model was considered. Mass and energy balance equations for the adsorbent bed, condenser and evaporator are presented in the following equations:

푀 푐 + 푀 푐 퐶 + 푀 푐 = 푚̇ 푐 푇 , − 푇 , + Δ퐻 − 푐 푇 푀 (1)

푀 푐 + 푀 푐 퐶 + 푀 푐 = 푚̇ 푐 푇 , − 푇 , + Δ퐻 − 푐 푇 푀 (2)

푀 , 푐 + 푀 , 푐 , = 푚̇ 푐 푇 , − 푇 , + Δ퐻 − 푐 푇 푀 + 푐 푇 푀 (3)

푀 , 푐 + 푀 , 푐 , = 푚̇ 푐 푇 , − 푇 , + Δ퐻 푐 (푇 − 푇 ) 푀 (4)

Provided that there is no heat recovery processes, a theoretical COP of the adsorption cycle can be defined as the ratio of the latent cooling energy of

refrigerant to the sum of the heat input during the switching and the desorption processes.

퐶푂푃 = ( )( ) ( )( ) (5)

In Figure 2 COP variation is presented like a function of heat source temperature. It is seen that COP of the adsorption cycle increase with the increase of the of heat source temperature. The COP of the adsorption system first increases, reaches a maximum value and then decrease slowly with

increasing hot water temperature because of the size limitation in utilizing the thermal heat of high temperature hot water.

Because of the small COP for these machines, their implementation on a large scale is a barrier as compared with traditional mechanical vapour

Computer Applications in Environmental Sciences and Renewable Energy

ISBN: 978-960-474-370-4 29

compression systems where the COP is 10 times bigger so must be carried out a more complex analysis, to improve the system.

Fig. 2. COP variation as a function of Heat Source Temperature in adsorption systems

2.3 Solar cooling using adsorption refrigeration

Adsorption refrigeration cooling machine is an alternative-periodic system in the sense that the two devices G1/A2 and A1/G2 work alternative/periodic, each with dual role of adsorber and vapour generator.

Fig. 3. Solar cooling adsorption refrigeration alternative/periodic system

A1,2/G1,2 – adsorber 1,2/generator 1,2; C – condenser; E – economizer; V – evaporator; AHU –

air handling unit; VR – adjustment valve; R – hot water tank; K – gas boiler; P – circulation pumps

2.3.1 Working principle - Period I

Vapor generator G1 has a surface rich in refrigerant vapor, water and by heating with heat from the hot water from solar collectors/boiler with temperature ~ 80-90oC, ammonia vapor is released and moves to the condenser C. The condenser condenses vapor, liquid subcooling results in the economizer E and then the liquid is laminated in VR adjustment valve. After the fluid enters the evaporator V, where vaporizes, cooling the water that serves air conditioning AHU. Vapors from V are adsorbed in A1 adsorber surface, the heat of

adsorption being taken over by a circuit cooling water from the cooling tower TR. The condenser is cooled by recycled water throughout the TR. 2.3.2 Working principle - Period II

Adsorber A1 become generator G2 and adsorber A2 become generator G1 and after the operation is identical to that in the period 1. 3 Absorption refrigeration system

During the absorption cooling process, compression is achieved by using a second fluid which is capable of absorbing the main refrigerant circulating through the other three components of the cooling system. Exiting the absorber, in order to achieve the absorption process, heat is extracted into the environment. This process results in a homogeneous liquid solution which is pumped into the generator. The process inside the system generator consists in separating the two fluids by external heat input. In an absorption cooling system, the compression work is much smaller than that in the mechanical vapor compression cooling systems as the compressed working fluid is a liquid and not vapor. On the other hand, in the generator a large amount of heat at high temperature (usually above 100C) is produced. This leads to lower coefficient of performance COP of an absorption system at sub-unitary values, usually 0.7. COP can be increased by using waste heat or solar energy. Figure 4 illustrates a schematic diagram of an absorption refrigeration system.

Fig. 4. Schematics of an absorption system 3.1 Classification of absorption cooling systems Absorption cooling systems are widely used in air conditioning equipment. They can be classified according to several criteria.

Computer Applications in Environmental Sciences and Renewable Energy

ISBN: 978-960-474-370-4 30

1. According to the working fluid. The fluids most widely used in absorption systems are LiBr-H2O (water is the cooling agent and lithium bromide the absorbing fluid) and NH3-H2O (ammonia is the cooler and water the absorbing fluid). LiBr-H2O solution is used at positive temperature (because water which is the refrigerant, freezes at 0C at atmospheric pressure) while NH3-H2O solution can also be used to obtain negative temperatures. However, water-ammonia solution systems are not used, because of a low performance coefficient (COP average value is 0.6), require larger areas of heat exchange and have a high initial cost. Current research is oriented to discover other working fluid pairs able to work in absorption cooling systems. 2. According to the number of effects. The number of effects describes the numbers of refrigeration cycles connected in cascade. A single effect cooling absorption system works according to a refrigeration cycle, while a double effect system uses heat from the absorber of the high pressure stage in the low pressure regenerator. In this way, the energy introduced in the system is used twice and the average COP of the refrigeration cycle is doubled (usually COP = 1.4 for double effect system vs. COP = 0.7 for single effect system, if the solution is LiBr-H2O). Cooling systems with single absorption effect can use hot water with temperatures around 80 C, while the double effect cooling systems require water or steam at temperatures of 120 C . 3.2 Mathematical model for an absorption system The absorption refrigeration modeling is based on the energy and mass conservation equations. In order to analyze the absorption refrigeration system, mass, component and energy balance must be performed for each system part like below. For evaporator: 푚̇ = 푚̇ = 푚̇ (6) Φ = 푚̇ (ℎ − ℎ ) (7) For the expansion valves: 푚̇ = 푚̇ = 푚̇ , ℎ = ℎ (8) 푚̇ = 푚̇ , ℎ = ℎ (9) For the generator: 푚̇ = 푚̇ + 푚̇ (10) 푚̇ 푥 = 푚̇ 푥 + 푚̇ 푥 (11)

Φ = 푚̇ ℎ + 푚̇ ℎ − 푚̇ ℎ (12) The performance coefficient of the system is: COP = (13)

The performance coefficient, COP, depending on the boiling temperature presents a maximum around the value of 90°C for the evaporator temperature tvap=-3°C, after which it uniformly decreases. 3.3 Solar cooling using absorption refrigeration

Using solar energy for air conditioning-SOLAR COOLING is one of the most important topic for research, Heat Engineering Chair from the Technical University of Civil Engineering Bucharest UTCB being involved in such a research. Solar driven refrigeration systems are absorption type or mechanical compression type. As refrigerants/absorbent pairs the absorption systems use LiBr/Water or Ammonia/Water. A 17 kW, YAZAKI, LiBr/Water system was installed inside the laboratory of the Faculty as shown in Figure 5.

Figure 5. Solar Cooling installation diagram Legend: 1 – flat solar collectors; 2- plate heat

exchanger; 3 – warm water tank; 4 – boiler; 5 – distributor; 6 – absorption chiller Li Br/water; 7 –

cooling tower; 8 – circulators; 9 – hydraulic unit;10 – hydraulic unit control; 11 –chilled water consumer for air conditioning; 12 – hot water consumer; TC –

temperature sensors. The operation of the experimental setup: the 10%

ethylene-glycol/water mixture circulating through the solar collectors, 1, transfer heat to the water from the 4000 l tank 3, by means of the plate heat exchanger 2. The hydraulic unit 9, is controlled by the temperature sensors TC: only positive temperature differential between solar panels and storage tank keep in operation the circulator. The vapor generator inside the absorption chiller is drove by the hot water prepared in the tank 3. The system

Computer Applications in Environmental Sciences and Renewable Energy

ISBN: 978-960-474-370-4 31

was designed for bivalent operation: if the solar radiation is not sufficient to heat the water at the temperature requested by the vapor generator then a back-up boiler 4, supplements the necessary thermal energy. The distributor 5, prepares the water at the requested temperature for the vapor generator. To improve the efficiency of the solar collectors the temperature of the hot water leaving the generator has to be lowered at approx. 30…35 °C, which is realized by the plate heat exchanger 2.

The chilled water, approx. 6 °C, is stored in a collector before being used in the air conditioning system.

The research team from Civil Engineering Faculty/Building Services Department proposed an air-conditioning/ventilation system to be used in the University Auditorium and also for the New Campus, the Research Center PRO-DD of the Transilvania University in Brasov. Figure 2 shows the air-conditioning diagram for the Auditorium located in the central Library, which consists of a 90 kW absorption LiBr/Water system powered by solar energy together with a 60 kW mechanical compression system PV electrical driven.

Figure 6. The air-conditioning diagram of the Auditorium and the central Library The system proposed for the Research Center is presented in Figure 7: for air conditioning during the summer an absorption LiBr/Water SOLAR COOLING was proposed, and for the winter a mechanical compression heat pump having the soil as source and a water coil is working bivalent.

Another issue was analyzed for the SOLAR COOLING, i.e. Ammonia/Water and Figure 4 presents the proposal offered to ARIZONA STATE UNIVERSITY in USA. Considering the high temperature and low humidity climate we have analyzed the operation limits of the one stage/two stages systems.

Computer Applications in Environmental Sciences and Renewable Energy

ISBN: 978-960-474-370-4 32

4 Environment Protection The environmental protection by using natural refrigerants is a worldwide concern reflected by the scientific research. After the MONTREAL Protocol the HCFC refrigerants have been replaced by HFC ones. The Global Warming Potential GWP is presently the index taken into consideration when analyzing refrigerants. The greenhouse effect produced by gases, is compared with the CO2, considered as having a GWP=1. Refrigerants escaping into the atmosphere have a very strong effect in terms of global warming. For example, 500 kg of refrigerant R134 escaped into the atmosphere during one year has the same result as the exhausted gases from a car after 2.5 million miles.

Figure. 7. Summer process Legend: G – generator; C – condenser; E –

evaporator; A – absorber; 1 – solar collectors: 25-30 kW; 2 – hot water boiler Q=25-30 kW, when

collectors do not work; 3 – heat exchangers 30 kW; 4 – absorption cooling machine BrLi/H2O Yazaki Qo=17 kW; 5 – dry cooling tower; 6 – cold water

storage tank, T=12…15 oC, V=2m3; 7 – vapor compression cooling machine Qo=17 kW.

Coefficient of performance for absorption C.M. COP=0,6

Coefficient of performance for vapor compression C.M. COP=2,5-3 (powered with electricity)

Figure 8. Ammonia-water absorption Solar Cooling

Legend: PS – flat solar collectors; Pc- circulating pumps; R – warm water tank; G – generator; A – air

cooled absorber; V – evaporator; C – air cooled condenser; E1,E2 – economizers; VR1,VR2 –

expansion valve 1,2; Co – consumer; 5 Conclusions

Adsorption/absorption heat pumps and chillers can save considerably fuel by using renewable heat sources or waste heat available at temperature that is low enough. A reduction of the carbon dioxide emission together with a global warming potential is resulting. In fact adsorption/absorption systems exchange heat with three thermal reservoirs contributing to overall energy efficiency. The use of an adsorption/adsorption chiller during high summer-cooling demand periods or even in normal operating hours is economically beneficial especially in case of a favorable cost ratio of electricity to natural gas or by using the solar energy. References: [1] X1. Serban A., Florea C., Boian I., Nastase G.,

The role of natural refrigerants in future refrigeration and heat pump systems, Recent Advances in Inteligent Control, Modelling and Computational Science, No.16, 2013, pp. 73-77.

[2] X2. Serban A., Boieriu L., Nastase G., 2nd International Conference on Energy and Environment Technologies and Equipment, Brasov, 2013.

[3] www.dralexandruserban.ro

Computer Applications in Environmental Sciences and Renewable Energy

ISBN: 978-960-474-370-4 33