ALONE. small scale solar cooling device Project No TREN FP7EN Project No TREN/FP7EN/ ALONE. small scale solar cooling device

Transcription

1 Project No TREN/FP7EN/ ALONE small scale solar cooling device Collaborative Project Small or Medium-scale Focused Research Project DELIVERABLE D5.2 Start date of the project: October 2008, Duration: 48 months EURAC Co-Ordinator of the Responsible Beneficiary: Maurizio De Lucia Authors: Davide Fissi Status: Draft Co-Ordinator of the Project: Maurizio De Lucia 1

2 Summary 1. Introduction System Planning... 3 a. Thermally driven chiller... 3 b. Solar collectors... 6 c. Storage... 8 d. Heat rejection... 9 e. Pumps... 9 f. Backup System Design choices

3 1. As the technology is still in an early phase of application, well established roles of thumb, design graphics and dimensioning standards do not exist yet. The most relevant aspect is that the three main values for a SHC system, solar radiation, external temperature and internal loads, are not constant, so it is not easy the definition of the design lines. In the follow some advices about the main components of a low temperature solar heating and cooling system are shown. 2. a. Thermally driven chiller The term chiller denotes a technology that transfers thermal energy from a low temperature level to an higher. Such transfer requires a thermodynamic input either in the form of heat or mechanical work. Compression chillers use an electric driven compressor to rise the pressure (therefore the temperature) of a fluid evaporating under the action of the body being chilled. The compressed vapour can in this way reject the heat yield by the chilled body to an higher temperature sink (ambient air typically). Sorption chillers strongly reduce the electricity consumption to engender such a process (by a factor 2 or more), by eliminating the compressor and using a couple composed by an evaporating fluid and a material that easily absorbs (fluid material) or adsorbs (solid material) the fluid. The temperature rise is this time naturally produced by the adsorption process or by compressing the mixture vapour + (ab)sorbent with a with a pump, with a much lower electricity consumption. Given the limited amount of sorbent, the sorption process cannot be extended indefinitely; the sorbet has to be regenerated (the vapour has to be expelled by the sorbent) through high temperature external thermal energy. Fig. - 1 Absorption chiller scheme 3

4 Fig. - 2 Schematic principle of thermal driven absorption chiller and performance indexes of a heat driven refrigerator machine So the absorption/adsorption machines pick up heat from the high temperature source (the solar field), pick up heat at low temperature (effectiveness) and return heat on medium-low. Fig. - 3 Absorption chiller - concept According to the diagram above it is possible to define the efficiency of the chiller as a function of the heat at high temperature and of the heat provided to users at low temperature. 4

5 Fig. - 4 Absorption chillers efficiency The COP of the absorption machines strongly depends on the type of machine used: single-effect machines work with the COP around 0.7 (use the external source only once in the thermodynamic cycle), double-effects machines have COP variable from 1.1 to 1.4 (can be seen as two single-effect cycles working in a cascade and using two external sources at different temperature levels), while machines can have triple effect of COP also 1.6 or higher. Machines with double or triple effect (still very rare), in addition to higher COP, sometimes allow to reach lower temperatures, on the other hand require that heat supply at a higher temperature: for single-effect machines temperatures below 100 C are sufficient, while for double-effects machines at least 140 C are approximately required, depending on the different models. The triple-effects machines require temperatures higher than 200 C: until now only the Thermax India realized a pilot plant with an absorber of solar cooling triple effect. Fig. - 5 cop of the various ab/adsorption technologies discussed A number of products are available on the market using different couples and external thermal energy at different temperature levels. In following tables a brief list of products actually available on the market is presented. 5

6 Table 1 Comparison of different market available small capacity absorption chillers b. Solar collectors Solar thermal energy is a relatively cheap, it is suitable for regenerating the sorbent in a sorption chiller and there are several different collector technologies available on the market Which solar collectors type is best suited for cooling devices depends on the operative temperature of the application. 6

7 FPC ETC CPC PTC FLC Global radiation Global radiation Global radiation Direct radiation Direct radiation T max 90 C T max 100 C T max 110 C T max 200 C T max 200 C Table 2 Examples of solar collectors technologies (Sources Wagner Solar, Riello, Soltigua) Flat Plate (FPC), Evacuated Tube (ETC) and Compound Parabolic collectors (CPC) can produce hot water at maximum temperatures between 90 C and 110 C, with harvesting efficiencies (ratio between thermal power at collector outlet and incident radiation) in the range of 40%-50%; above this limit of temperature the efficiency drops down quickly to zero. Parabolic Trough (PTC) and Fresnel Linear Collectors (FLC) easily warm up thermal oil or steam at temperatures up to 200 C, with the same conversion efficiencies. The plus of the latter technology is paid with the fact that concentrating collectors only harvest the direct solar irradiation, whilst FPC, ETC and CPC capture both direct and diffuse (reflected) radiation. As a consequence, PTC and FLC are more suited for installation in dry, low latitudes locations. FPC, ETC and CPC can also be exploited at places with higher cloudiness and latitudes. The selection of a solar collector according to the type of application has to be undertaken considering that the operating temperature raise up decreases the efficiency of the collector, but increases the COP of the chiller. A balance must be found by choosing the right type of solar field and chiller. Fig. - 6 Solar collector and working temperatures for different application 7

8 Fig. - 7 Collector efficiencies at different temperature differences The coupling of a sorption chiller to solar collectors has to be decided on the basis of the chiller rated regeneration temperature and collectors operational temperatures. Flat plate and ETC are suitable for coupling with single effect adsorption chillers. CPC are more suited to be coupled with single effect absorption chillers concentrating collectors (PTC and FLC) can be matched with double effect chillers. A simple pre-sizing of the solar collectors area compared with the chilling power of the sorption chiller can be elaborated for flat collectors (FPC, ETC and CPC) on the basis of the following relationship: Example: in north Italy, an average value of global radiation (G) in summer on a 30 tilted surface is 0,8 kw/m2; using an CPC with an efficiency of 50% (ηdesign) and a sorption chiller with a COP of 0,6, the collectors area is around 4 m2 per kw of chilling power. To date, almost all plants of solar cooling using single effect absorption chillers, this mainly for the most low cost of the flat plate collectors evacuated or stationary CPC, ensuring sufficient temperatures to operate with single effect absorption machines. These systems have, however, a major limitation, bonded to the surface request from the collectors, not always available. In this context, the use of double effect absorption machines and therefore and collectors at medium temperature, has the considerable advantage of requiring a lower surface. In fact, considering a relationship between the COP of a machine in a single effect and double effect, it has: At the same plant performance and efficiency of the collectors, a double-effect absorption machine saves more than 40% of the area needed for collectors fueling a single-effect chiller. c. Storage The heat storage water tank plays a key role in planning a solar heating and cooling system exploiting flat collectors (FPC, ETC and CPC). Products assuring the best stratification of the thermal energy have to be selected. Moreover 8

9 the sizing has to grant an optimization between the energy stored to shave irradiation peaks and valleys and the reactivity to the system requests. As a results of the analysis of more than 60 solar heating and cooling installations within the IEA SHC Task 38, it is recommended to install a hot storage volume of 50 to 75 l/m² collector aperture area, to satisfy a sufficient fraction of the heating, cooling and DHW loads. Today, it is not economic feasible to store thermal energy at temperatures higher than 100 C. However, technologies using phase change materials to store energy at higher temperature and be coupled with concentrating collectors are in a prototypation stage. d. Heat rejection It is suitable foresee more than one cooler devices. In order to reduce management cost the control strategy has to prefer the device with the lowest electrical consumption. Heat rejection management is another crucial issue in a solar heating and cooling system: a 10 kw compression chiller rejects around 14 kw of heat towards the ambient, whilst a sorption chiller with the same chilling capacity rejects around 30 kw of heat. Different heat rejection technologies can be used with respect to sorption chillers: Dry cooling tower Wet cooling tower (open or closed circuit) Ground or ground water heat exchangers The technology assuring the lowest electricity consumption has to be preferred, compatibly with the planning requirements. Energy efficiency class of fans shall be checked. Management strategies have to be anyhow implemented minimizing the pumps and fans electric consumption Possibly the rejected heat shall be reused for example for warming a swimming pool (single effect chillers) or within the post-heater of an Air Handling Unit. e. Pumps It is a good practice to install variable speed pumps. Energy efficiency class of pumps shall be checked. In the solar collectors circuit the pump speed might be varied with solar radiation and temperature difference between water at collectors outlet and storage tank. In the heat rejection circuit the pump speed can be adjusted depending on the punctual thermal loads. f. Backup System In a solar heating and cooling system, the use of a fossil fuel boiler as a backup of external thermal energy to the sorption chiller in place of solar energy is to be avoided with respect to single effect devices, due to their low thermal COP (lower than 1). Therefore, an electric driven compression chiller has to be preferred as a cold backup. On the other hand, a warm backup, as a gas boiler, is a suitable option with respect to double effect chillers due their COP higher than 1, to cover cooling loads when solar radiation is scarce or missing. Some double effect chillers are available on the market with integrated gas burner. The integration of all the components might be a very complex exercise compared to the planning of a traditional heating and cooling system. Therefore a careful design of the system has to be performed tackling the correct sizing of the components and the elaboration of an integrated control strategy, for all the elements to perform consistently and efficiently. To this purpose, the selection of a prefabricated Control Unit can be extremely beneficial. The degree of prefabrication varies from supplier to supplier. It is important that the controller is included in the package and the control strategies have been pre-defined by the manufacturer. 9

10 Not all packages on the market include a control system that manages, besides the solar thermal plant and the chiller, the thermal energy distribution to the building: in these cases, a separate controller is needed. However, the overall control concept of the building often has a big influence on the performance of the entire system. Therefore packages that include the control algorithms also for the distribution system are recommended. Since the solar irradiation availability has an erratic behaviour, the design of the system cannot be executed on the basis of maximum building loads and components rated performance, because it could result in a significant underperformance with respect to the expectations. A seasonal energy performance evaluation has to be assessed by means of simulation tools accounting for the full dynamic behaviour of the components and system as a whole. 3. Optimal design involves a large number of choices in terms of setting priorities and trade-offs. Now we define some parameters to be taken into account during the design phase we need to weight the various strategic choices in the design phase. SF -> Solar Fraction Percentage of end-user energy demand met by solar energy. SN -> Solar system efficiency Ratio of the annual value of energy produced by the solar field and the annual value of global radiation active on the solar field area. SE -> Specific solar energy yield Ratio of the annual value of energy produced by the solar field and the value of the reference area for the solar field. The maximization of SN involves the minimum size of the solar field in order to minimize excess solar energy unused leads to a reduction of SF. The maximization of SF involves an increase of the size of the solar field and then an increase of the storage system with consequent costs increase. Fig. - 8 Sizing is a compromise between cost and yield So a good design should include: a detailed analysis of all energy flows in the design phase, at an early stage of the design of economic feasibility 10

11 a software simulations for the definition of optimum sizes of the main components of a SHC system evaluate the thermal loads as closely related to the climatic conditions, air temperature, air relative humidity, solar radiation should be evaluated climatic conditions that affect on the performance of the solar field consider the weather conditions that influence the choice of cooling technology - dry/wet cooling the close connection between the distribution system, the solar collectors and the cooling technology the operation temperature, the thermal jumps Fig. - 9 Interdependence between climatic conditions and load condition and the employed technology for cooling and solar collector Fig Comparison among conventional chillers and solar assisted absorption chillers; PE is the primary energy consumption per unit of cooling effect A good design must also take into account the operation of the system in a wide range of off-design configurations. Assessing, for example, the behavior of the chiller to vary the input temperature and the cooling conditions. In order to ensure an adequate behavior of the plant even under off-design conditions, very probable in according to the variability of the solar radiation. 11

12 Fig Typical characteristics of absorption/adsorption machine Fig Typical characteristics of absorption/adsorption machine Fig Typical characteristics of absorption/adsorption machine 12