1 SOLAR COOLING WITH SMALL SIZE CHILLER: STATE OF THE ART F. Asdrubali, G. Baldinelli, A. Presciutti University of Perugia (Italy) - Industrial Engineering Department 14 th European Conference - Renewable energy in heating and cooling Edinburgh 21 st January 2011
2 Introduction The summer air conditioning demand is growing continuously, especially in residential applications; the correspondent demand of electric power may cause crisis of the electrical net. Absorption refrigerators represent an interesting alternative to compression machines, especially when waste heat or solar energy is available. The market is beginning to propose small-size absorption machines especially designed for residential buildings. The machines can be effectively incorporated into so called solar combi-plus systems, where solar energy is used to produce domestic hot water, space heating and cooling. A recognition of these solutions is presented, with particular emphasis on their performances when the thermal source is constituted by solar energy.
3 Feeding absorption machines with solar energy Main problems linked to solar fed absorption plants should be summarized as follows: strict plant dependence on environmental parameters such as external air temperature, solar irradiation and wind speed; high initial cost; low collectors efficiency opposite to best refrigerator performance at high temperatures; temporary losing of the ideal coupling between the sun and the absorption machine (hot days with scarce irradiation, morning or late evening cooling load and sunny days without cooling demand).
4 Machine A Water-LiBr single stage absorption cycle 11 kw absorption chiller Heat removal by cooling tower Heat recovery between absorber and generator Electrical solution pump between low and high pressure environment German manufacturer
5 Machine B Water-LiBr single stage absorption cycle 10 kw absorption chiller Heat removal by cooling tower Heat recovery between absorber and generator Electrical solution pump between low and high pressure environment German manufacturer
6 Machine C Water-LiBr single stage absorption cycle 15 kw absorption chiller Heat removal by cooling tower Heat recovery between absorber and generator Bubble pump between low and high pressure environment Japanese manufacturer
7 Machine D Water-LiBr single stage absorption cycle 5 kw absorption chiller Heat removal by cooling tower or integrated dissipation device Rotating generator Spanish manufacturer
8 Machine E Water-LiCr triple stage absorption cycle Intermittent functioning 4 kw absorption chiller Heat removal by cooling tower 4 internal circulation pumps Swedish manufacturer
9 COMPARATIVE ANALYSIS Data from the manufacturers rating and functioning curves; same working conditions, as far as possible; machines feeding solar collectors, fixing therefore the following values: generator inlet temperature T g,i = 85 C; outlet chilled fluid temperature T c,o = 9 C; tower outlet cooling fluid temperature T t,o = 30 C.
10 COMPARATIVE ANALYSIS Performance parameters analyzed Chilling power; thermal Coefficient Of Performance (the ratio between chilling power and heat given to the generator); global Coefficient Of Performance (the ratio between chilling power and the sum of the heat given to the generator plus the electric energy absorbed).
11 COMPARATIVE ANALYSIS Performance parameters analyzed Electric energy consumption Generator-absorber pumps (when applicable); generator rotation (only for sample D); pumps for the circulating fluids of the cooling tower and the solar circuit (20 W/kW of fluid transported); engine of the evaporative tower (10 W/kW).
12 Position Chilling circuit Heating circuit Cooling circuit Paramete r Results in fixed conditions M.U. Sample A Sample B Sample C Sample D Sample E Power kw T c,o C Power kw T g,i C Power kw T t,o C Electric absorption kw Thermal COP Overall dimensions Global COP Weight kg Length mm Width mm Height mm
13 Samples chilling power as a function of outlet chilled temperature T c,o (T t,o =30 C, T g,i =85 C) Chilling power (kw) Machine outlet chilling temperature ( C) Sample A Sample B Sample C Sample D Sample E
14 Samples Coefficient Of Performance as a function of outlet chilled temperature T c,o (T t,o =30 C, T g,i =85 C) 0.75 COP Sample A Sample B Sample C Sample D Sample E Machine outlet chilling temperature ( C)
15 Three main reasons still prevent solar cooling (only chilling) and solar combi-plus system from spreading in chiller and air conditioning market: difficulty to identify a standard configuration for single chiller; low numbers of machines produced and high costs. absence of knowledge of the system behavior. The definition of standardized system configurations might reduce considerably the design effort for each single application and it could be the basis for the development of package solutions manufactured with the advantage of the large scale production.
16 Efforts for standardization Within the Intelligent Energy European project 2007 Solar combi+, an extended campaign of numerical simulations has been implemented, with TRNSYS code, on two basic plant configurations derived from economical and technical analysis. Within the basic systems, a number of parameters were varied: geographical location (Naples, Strasburg and Toulouse), chiller brand, collectors type (flat plate, evacuated tube collectors), chilled/warm water distribution system (fan coils and chilled ceiling) and other technical parameters. Results showed that the chilled ceiling, wet cooling tower and evacuated tubes collectors configuration allows the best performance.
17 Efforts for standardization Two parameters (Heating Degree Days and Cooling Degree Days) are used to evaluate the energy demand of a building, based only on the external temperatures.
18 Efforts for standardization It is showed that a unique configuration gives the optimum in terms of primary energy savings and costs for each country of Europe. The reason is easily explained considering that that lower is radiation value, the lower is the chilling load; wider collector surfaces need further investments, that are not justified by the relative better performances
19 Cost analysis A detailed market survey showed that the cost of small-size absorption and adsorption chillers stands around per kw of chilling power; for the solar collectors euro/m 2 are needed, so the entire system has a cost range between and per kw of chilling power. Typically, the whole cost is split as follows: 1/3 chiller, 1/3 collectors and 1/3 the rest (pipeline, pumps, control systems and cooling tower). Naturally, some variations are found among the various installations all over Europe, mainly because of different prices of solar collectors.
20 Cost analysis If a simple economic analysis is conducted through a comparison with conventional cooling heating systems (estimated cost equal to , with similar performance), it is possible to evaluate the Payback Time. The extra cost is , while the saving is given by the value of the electric energy not used to produce chilling power: /year. Thus, the simple Payback Time (no maintenance, no energy cost increase and no inflation) is about 20 years, while it diminishes to approximately 10 years if at least 50% of the investment is covered by incentives.
21 Conclusions A recognition of a series of small size absorption refrigerators fed with solar energy is reported. The analyzed machines cover different chilling powers (from 4 to 15 kw) and have different working principles and design. In terms of COP, machines show generally similar behavior. The sustainability and feasibility of this technology are confirmed. The higher margins of improvements consist of the costs abatement.
22 Conclusions A recent European Research Project has fixed the main characteristics for such typology of plans witch are defined in an optimal solar collector surface and storage tank volume. Such parameters are usable for the different European Countries. Up to now solar combi-plus can have a payback time above 20 years without incentive but the decreasing of price of solar collectors and of absorption machine, along with State incentives, could help the payback time to reach value less than 6-8 years.