Category Archives: EuroSun2008-10

Simulated system

There are several possible ways to connect a solar thermal plant to a DH network, with no proven general optimal system layout. The chosen system design principle is shown in fig 2., where the storage tank is connected in parallel with boilers. The control system keeps the outlet temperature from the collector array always above the required DH supply temperature by varying the collector loop mass flow rate. This control strategy is needed, if one wants to be able to completely turn off the boilers for a period during summer.

2.1. Studied system sizings

The main sizing criteria was to be able to turn off boiler(s) for some period during the summer, as this would both save maintenance costs and reduce boiler operation at low efficiency. This requires a solar fraction of around 90-95% during June-July. It should be mentioned that this is not the optimal case for the solar thermal plant as high solar fractions result in lower specific collector yields, mostly because of higher collector temperatures and the fact that the storage tank is more often fully charged. It was also decided that for simplicity boiler operation would not be considered in the storage tank sizing. The benefits of having a storage tank available for a boiler are too difficult to convert to economical benefits in a generalized manner, and can only be considered case to case when sizing a plant.


Подпись: Fig. 2. Simulated system layout and main simulation parameters. По (-) 0.8

a1 (W/m2K) 1.76

a2 (W/m2K2) 0.0323

A total (m2) 13.52

Aaperture (m ) 12 5

Storage tank

h/D ratio (-) 2.5

lave, internal (W/mK) 0.8

Insulation thickness (m) 0.3

DH flow temperatures

Tsupply/Treturn winter (°С) 90/50

Tsuppiy/Treturn summer (°С) 65/40

Tamb at sizing point (°С) -30

6. Feasibility

The scenario is that solar panel array and heat storage tank are added to an existing DH-plant. The panel array and storage tank are ground mounted. Expected main financial savings: fuel, maintenance and electricity. Cost functions for the investment, running costs and savings were based on earlier studies, reported costs from realized plants and budget prices from manufacturers. The NPV calculations were done and the results presented in a manner that tries to answer the question “Under which scenarios would it be feasible?” rather than more traditional case studies, where the feasibility for only a few scenarios are calculated and for which a sensitivity analysis is done. In the presented graphs a shortest feasible payback period for a scenario is found where the curve crosses the x-axis.

The useful lifetime of a solar thermal plant is estimated to be 20-30 years. No subsidies are considered in the calculations. At the time of the study, energy prices for large scale consumers were as in table 1. The calculated electricity savings were so small that changes in electricity price had insignificant effect on feasibility.

Table 1. Used energy prices.














It must be noted here that economic viability is up to the investor to decide, keeping in mind that different actors within the heat service business may have different decision criteria depending on company size and long term strategy. Therefore only comments on whether a feasible payback period can be found within the lifetime of the plant are given here.

Control of a domestic hot water solar heater with weather forecast

Mihai Radulescu1* and Aude Lepeltier2

1 EDF R&D Site des Renardieres, Dept EnerBat, Avenue des Renardieres — Ecuelles, 77818 Moret sur Loing
2 Ecole des Mines de Douai, 941 rue Charles Bourseul, BP 10838, 59508 Douai Cedex
* Corresponding author, mihai. radulescu@edf. fr


This paper presents the efficiency improvement and electricity economy obtained by introducing an advanced control of a hot water solar heater (HWSH) using a weather forecast. In order to quantify the energy gain of the advanced control a numerical model was built for the storage tank. This model takes into account all inlet and outlet energy fluxes and it’s able to determine the temperature inside the tank at any moment and height. The advance control is acting on the set-up temperature of the auxiliary electric heater in order to prepare a reduced quantity of hot water during night and to allow to the solar loop to recover as much as solar energy possible. The comparison between a standard system and a HWSH with weather forecast shows that a 7% annual electrical economy is found and a 3% efficiency improvement.

Keywords: solar thermal, hot water, control system, weather forecast.

1. Introduction

Electricity production in France is mostly nuclear with 88% completed by small parts of natural gas 4% and hydraulic 8%. The most important electricity producer is by far Electricite de France (EDF) with a total annual capacity of about 650 TWh, and just 490 TWh in France.

Face to global warming and approaching oil peak, research efforts turned toward renewable energy sources. Theses technologies can produce “clean” energy without or with low carbon dioxide emissions but remain still expensive and less widespread. One technology, with unlimited resources and a good potential to overcome the actual setbacks, is the solar thermal. Probably the main disadvantages leading to low solar energy use are the intermittent availability and the low reliability of the energy source. Transforming the solar energy in heat seams so simple, but to recover this heat and to use it efficiently becomes more complicated. Heat has to be carried by a fluid (air or water) to be used immediately or to be stored.

This paper focuses on domestic hot water solar heaters (HWSH) with a storage tank and forced circulation of the heat carrier fluid. Such a system is coupled with a meteorological device (not described) to provide the weather forecast. The energy economy and comfort gain are established with respect to a classical HWSH. The description of the system will be followed by a short explanation of the model. The results are divided in two parts: with and without the weather forecast.

Solar Combi + systems


Solar Combi+ systems use heat from solar thermal collectors to provide heating in winter, cooling and summer and domestic hot water all the year round. Fig. 1 sketches the main components, which make up a typical system: (i) the solar thermal collector to provide the heat might be backed up by another heat source, (ii) a storage tank can either be installed on the warm side, as drawn in the figure, on the cold side or on both, (iii) the domestic hot water tank might be included in the hot storage or be a separate tank, (iv) the sorption chiller is fed with hot water (70-100°C), (v) rejects heat at intermediate temperature (30-40°C) to a cooling tower (dry, wet) or another heat sink (as e. g. a swimming pool) and (vi) delivers chilled water to the cold distribution (be it a chilled ceiling, fan-coils or air handling units), whereas (vii) the heat distribution should possibly be a low temperature system.

Fig. 1. Example for the system components of a Solar Combi + System and project logo

All the single components of a system are now market available: But there is so far hardly a provider of system solutions, and that’s the point where the project is puts efforts forth.

Monitoring Results of a Solar Thermal System for. Hot Water Production in an Apartment Building

Jose Cavaco*, Pedro Horta, Joao Farinha Mendes,

INETI, Department of Renewable Energies, Campus do Lumiar do INETI, 1649-038 Lisbon, Portugal

* Corresponding Author, iose. cavaco@ineti. pt


Prior to recent building regulations enforcement, the use of Solar Thermal Systems (STS) in residential buildings was dependent on the proactive role of building promoters, as is the case of EPUL (Public Company of Urbanism of Lisbon), which has studied along the last decade, after a joint work with INETI, the possibility of using solar energy in new apartment buildings.

Such a STS was installed in a new multi-storey building just before the new building
regulations, making the use of solar energy in residential buildings mandatory, was enforced.

At the present, INETI is monitoring the STS over a two year period. The monitoring results, reported in the present article, make this system a good example for demonstration purposes, either from technical solutions and operation points of view, having in mind the need (because of solar obligation) and interest (for building certification) for the adoption of such systems in multi-owned buildings. [13]

According to the agreement between the two entities, INETI also assisted EPUL on choosing the solar thermal system (STS) supplier, on the detailed project approval and on the system monitoring for a period of 2 years, to have an idea of system efficiency in comparison with initial estimations and its follow-up for a period of time considered long enough to be sure about its behaviour in the future, giving also the opportunity to detect problems that can be solved in this initial period.

This paper presents a description of the solar thermal system configuration in point 2, and a description of the implemented monitoring scheme in point 3. The data already obtained will be shown and analyzed in point 4, as well as the actual dwelling owners’ degree of satisfaction. Present conclusions and remarks are presented in point 5.

Experimental Apparatus

A schematic diagram is shown in Figure 2, depicting the main components of the ISAHP test rig. Table 1 lists the instrumentation and monitoring equipment used in the experiment, whose part numbers correspond to the schematic diagram.

Table 1

List of instrumentation used





Pressure/Temperature transducer (0-100 psi, Senstronics LTD)



Pressure/Temperature transducer (0-500 psi, Senstronics LTD)

F — 1

Ultrasonic flow meter (Emerson)

F — 2

Positive displacement flow meter (Oval Engineering)


Watt meter (ISW8001, Powertek)


Thermocouple (24 Gauge T-type, Omega)

Подпись: Fig. 2. Schematic diagram of the ISAHP Experimental rig
The main components of the apparatus include: a nominal 1/3 HP single speed compressor, a thermostatic expansion valve, two flat plate counter-flow heat exchangers acting as the evaporator and condenser of the heat pump, a standard residential hot water tank (270 L), a variable speed pump and an auxiliary heater for simulating the solar collector heat input. R-134a was used as the working fluid for the heat pump cycle, and a 50 / 50% glycol/water solution by volume was used in the collector loop.

The collector loop operates in a similar fashion to that of a typical solar domestic water heating system. First, the glycol solution is pumped through the collector, or in this case the auxiliary heater, in a closed loop. The glycol solution absorbs energy through the “collector”, and a heat exchanger is used to extract the heat from the glycol, which acts as the evaporator of the heat pump loop. The R-134a superheated gas exits the evaporator and passes through the compressor, increasing in both pressure and temperature. The refrigerant then releases its heat and condenses through the natural convection heat exchanger, which delivers hot water to the storage tank. The R-134a liquid then passes through the thermostatic expansion valve, reducing its pressure before re-entering the evaporator.

. Cost of solar collector area

To investigate the influence of the collector costs on the cost/benefit ratio and the dimensioning of the system, the specific collector price was gradually reduced to 70% of the base case. Such a reduction could be realistic with improved materials and/or production processes. All other parameters match the base case. Figure 3 (a) shows that the less expensive collectors lead to higher primary energy saving, due primarily to an enlargement of the absolute collector area in the optimal system configuration. In opposition to the enlargement of collector area the storage device capacity almost stays constant within the examined range of collector costs, which means that the specific storage volume decreases. Selected results of the best configurations for the respective collector costs are listed in Table 1.

Case 1 feasibility

The feasibility and sensitivity analysis for Case 1 with pellets and oil as fuel are shown in fig 3. With today’s pellet prices a feasible payback period cannot be found during the plants estimated lifetime. With annual pellet price increases of between 5-10% the feasible payback periods start to come within


the range of the estimated lifetime. With today’s oil prices a feasible payback period can be found in Sweden but not in Finland, where an annual oil price increases of between 2-5% would be needed.

System description. Solar collectors

Two solar glazed panels heat up the heat carrier fluid of the solar loop. The absorbing total surface of the two collectors is 4 m[10] [11]. They are facing south (azimuth angle 0°) and have 45° with the ground. Neither roof integration nor shadows are considered. Collector parameters are: optical coefficient B = 0.81, firs order thermal coefficient k1 = 3.61W/m2K and second order thermal coefficient k2 = 0.0045 W/m2K2 . Stagnation temperature is 215°C’.

2.2 Storage tank

A 300 liters tank is used to storage the solar energy (figure 1). At the bottom of the storage tank an integrated heat exchanger supplies solar energy recovered by the panels. This exchanger is installed inside a stratifier high as 90% of the tank. The tank has also an auxiliary electric heater allowed to run between 22.00 and 6.00 if the upper volume didn’t reached the set-up temperature2. The upper volume heated by the auxiliary heater is 100 liters. No stratification is possible in the volume above the heater when in operation [1].

Tap hot water is delivered at 50°C by a thermostatic mixing valve. This one add cold water to the hot water from the tank.



S teiage tank



E iectric heater


C ontrol device


C oU w ater




Fig. 1. Layout of a domestic hot water solar heaters (HWSH).

2.3 Control device

A classic control device is an electronic box able to take two decisions:

• To run or stop the pump of the solar loop depending on the temperatures registered in the collector and at the bottom of the tank;

The new control device is able to decide also the set-up temperature of the auxiliary heater depending on the needs and on the weather forecast [2, 3].

3. System model

Presented model is a finite difference one with nodal discretisation, and is based on the mass and energy balance. The simulations are carried on by dividing the tank in many isothermal elements or slices, with equal volumes. For each element we take into account the next heat transfers and energy interactions (figure 2):

• Heat lost to environment,

• Heat exchange with n-1 element,

• Heat exchange with n+1 element,

• Energy input from the electrical heater,

• Energy input from the solar exchanger,

• Energy input from the n+1 element when hot water is poured,

• Energy output to n-1 element when hot water is poured.


Addressed barriers and offered solutions

The most important barriers for a broad application of small scale combined solar heating and cooling systems — further on called Solar Combi+ — and the solutions proposed by the project to overcome them are as follows:

1. Combined solar heating and cooling needs specialised design in order to make the single components play together optimally. So far every single system is designed from scratch. This (i) leads to a financial effort, which is not feasible for small applications, as design costs become prohibitively high in relation to hardware costs and (ii) often might overstrain the solar thermal installer, who would in most cases be the provider of the system to the end-user. Today there are no design guidelines for small scale systems and very few, yet not validated, package solutions on the market.

Virtual case studies will overcome this gap: Promising configurations will be identified, simulated for different typical conditions (i. e. utilization, climate, building type) and finally economically and ecologically rated. Out of the large number of virtual case studies, a small number of standard system configurations, which work best under different conditions are identified. Based on these, small scale sorption chiller and solar thermal industry will be able to provide consistent package solutions. These will enable planers and independent craftsmen to install reliable systems.

2. Small scale sorption chillers are expensive as production volumes are currently low.

The economical and ecological rating of the above described virtual case studies will allow identifying the most promising markets, where systems are yet at the edge of economical breakeven point or beyond, compared to traditional solutions. Accordingly tailored promotion and market strategies will considerably trigger the application of the technology. Following economies of scale will make small scale combined solar heating & cooling less expensive and thus viable in a broader range of applications and climates.

3. Small scale combined solar heating & cooling is not well known by traditional small scale solar thermal installers, planners, architects and potential clients.

Tailored dissemination plans include among other measures the training of solar thermal installers, targeted presentations to professionals, information of the public in most promising regions as well as advice to policy makers and promotion of pilot plant installation to public authorities.

Description of the system

The STS consists of 32 solar collectors with unit aperture area of 1.86 m2. The solar field is divided into 8 groups of 4 collectors oriented both at 27° (4 groups) and -63° (4 groups) azimuths, given architectural and space constrains. The tilt angle is 35°, optimized for the building latitude.

The effect of the decoupled orientation from south was studied on a daily basis and found never to exceed a 12% penalty (regarding an optimal south facing orientation) in monthly average (value for December, the less favourable month, being for example 8% in March, 1.35% in April, 0.87% in May and 0.16% in June).

The feeding of the collector batteries facing east and west is independent, thus using two separate lines starting in the pumping group (located in the basement, -1 floor).

After passing the two parallel collector fields, each one composed with 4 parallel groups of 4 collectors in series, the thermal fluid shares the same pipe which in turn feeds 5 vertical lines going to the independent building sectors feeding the individual storage tank located in each apartment.

Along its course, each line feeds 4 dwellings, until the technical room is reached. At the end of these lines there are flow meters and regulation valves to assure the hydraulic balance.

The heat exchange from the primary system to the secondary is done via a heat exchanger inside the storage tank located inside each dwelling. When the temperature inside the individual tanks, is higher than temperature of the primary water circuit, a three way valve was projected to avoid the heat transfer in the wrong direction.

The system has 4 pumps, 2 for each azimuthal group of collectors working in turns, so that the pumps can be repaired, if needed, without having to stop the system.

Upstream from the pumping group there is also the safety group, consisting in expansion vessel and a pressure safety valve, for each feeding line to the collectors.

The circuit includes also an energy count system, consisting of two temperature probes and a flow meter, that send information to the central which has a differential controller and calculates the energy delivered by the solar system. The "hot probe" is inserted in the totalizing pipe downstream after the two collector fields. The "cold probe" is placed just upstream of the pumping group.

Fig.1 is a schematic representation of the primary circuit.


Fig. 1. Schematic draw of this STSs primary circuit.