- The main components of the heating system
- Varieties and differences of collectors
- Solar energy problems
- Integration into the heating system
- Power calculation and installation steps
- Do I need a heat pump
- Solar heating system cost
Is it realistic to provide your home with solar thermal energy? Today we will discuss the prospect of using solar systems as the main source of heating, consider the issue of economic justification and efficiency of solar collectors.
The main components of the heating system
Solar collectors serve as the source of heating of the solar system, the purpose of which is the most efficient transfer of the energy of the infrared spectrum of solar radiation to the coolant. The thermal range of sunlight is 40–45% of the total radiative flux, in specific figures it is 200–500 W / m2 depending on latitude, time of year and day.
In principle, collectors alone are sufficient to build the simplest solar system. Through their channels, ordinary water can be circulated, used for household needs and home heating. However, this approach is not effective enough for a number of reasons, the first of which is the lack of replenishment of energy losses for a full day. Therefore, one of the most important elements of a solar heating system is a heat accumulator – a container with water.
House heating scheme with solar collectors: 1 – cold water supply; 2 – heat exchanger; 3 – heat accumulator; 4 – temperature sensor; 5 – coolant circuit; 6 – pumping station; 7 – controller; 8 – expansion tank; 9 – hot water; 10 – three-way valve; 11 – solar collector
The technical device of the solar collector is also a kind of limitation. Its channels have a rather small flow area, which gives rise to the risk of clogging with mechanical impurities. There is also a high probability of the coolant freezing at night, while the upper limit of the operating temperature range is 200–300 ° С. Collectors are designed for fast continuous circulation of the coolant, which comes in at a low temperature, is quickly heated by sunlight and just as quickly gives off heat to the battery.
Vacuum U-shaped solar collector tubes
For these reasons, it is customary to use propylene glycol with a set of special additives for direct heating in heat pipes. So, the third mandatory element of the heating solar system is a special coolant and an exchange circuit, which is often structurally included in the heat accumulator, or can be part of the collector itself.
Varieties and differences of collectors
Without going into the technical details of the device, the main difference between flat and vacuum collectors lies in the expediency of their use in different climatic zones. Flat collectors are best used in southern latitudes with prevailing temperatures above zero, vacuum collectors closer to northern ones.
The design of a flat solar collector: 1 – coolant outlet; 2 – collector frame; 3 – structured hail-resistant glass; 4 – absorber; 5 – copper tubes; 6 – thermal insulation; 7 – coolant inlet
The expediency of using certain types of solar collectors is due to a number of features:
- the inability of the vacuum collectors to clean themselves of snow on their own;
- high heat losses of flat solar collectors, growing with the temperature difference;
- low resistance of flat collectors to wind loads;
- high cost of the project on vacuum solar collectors;
- low temperature range effective use of flat collectors.
The design of the vacuum manifold with indirect heat transfer: 1 – inlet of the cooled heat carrier; 2 – heat exchanger (collector); 3 – hermetic plug; 4 – vacuum tube; 5 – aluminum plate (absorber); 6 – heat pipe; 7 – working fluid; 8 – heated coolant outlet; 9 – heat sink body; 10 – heat pipe condenser; 11 – insulation
One of the most important differences lies in the installation process. Flat-plate collectors require pre-assembled delivery to the roof, while vacuum collectors can be assembled on site. Also, flat collectors usually do not have their own heat accumulator and exchange circuit..
Solar energy problems
Solar heating systems are not without drawbacks, the most important of which is the inconstancy of the energy source. At night, the system does not heat up, and in prolonged cloudy weather, expecting a clear sky to heat up the house is a below average pleasure. If the battery, with a sufficiently large volume, is able to retain the required amount of heat at least until the morning, then several days of autonomous work in conditions of insufficient illumination can only be expected with a significant expansion of the solar farm. This, in turn, causes the opposite problem: when reaching the maximum power mode (for example, on a clear spring day), such a solar system will require more intensive heat removal or temporary shutdown of several absorbers with their shading.
It is important to understand that solar systems in the realities of the Russian climate cannot be used as the only or main source of heating. However, they are able to significantly reduce energy consumption during the heating season. Hybrid collectors work especially effectively, in which heaters are combined with photocells. If cloudiness delays most of the IR radiation, then the loss of the photoelectric part of the spectrum is not so significant.
Another disadvantage of solar collectors is the need for forced circulation of the coolant in the collector-accumulator system. Some vacuum collectors are equipped with a tank for natural circulation and located above the absorber. Such installations are usually used in hot water supply systems with water intake under the pressure of a cold water supply. But there are still ways to establish the joint operation of such solar collectors with a heating system..
Vacuum solar collector with tank
Integration into the heating system
There are two ways to combine solar collectors with an arbitrarily complex liquid heating system. The main source of energy can be either gas or electricity – there is no significant difference.
The first option is to heat the total daily battery. The accumulator communicates with the boiler jointly and sequentially; if the temperature is not high enough, the latter turns on and heats the liquid. A properly designed system of this kind can work effectively even without forced circulation..
1 – heating circuit; 2 – heating liquid; 3 – temperature sensor; 4 – pumping station; 5 – controller; 6 – pump; 7 – expansion tank; 8 – sanitary water; 9 – cold water; 10 – hot water supply; 11 – solar collector; 12 – heating boiler
The second type of combination involves the use of a heat accumulator with two circuits. Through one, heat is removed from the collector, through the second – heating of the coolant in the system, water from the battery serves as a source of hot water supply. Since the circuits are isolated from each other, more heat-absorbing fluids or antifreeze can be used in the heating system and the heat exchange cycle from the solar collector. The main disadvantage is the volatility of the system, because in both circuits the circulation is forced.
1 – cold water supply; 2 – temperature sensor; 3 – solar collector heat exchanger; 4 – boiler heat exchanger; 5 – collector coolant circuit; 6 – pumping station; 7 – controller; 8 – expansion tank; 9 – circulation pump; 10 – hot water outlet; 11 – heating boiler; 12 – solar collector
Power calculation and installation steps
The transition to solar energy does not accept haste and a superficial approach. Often, conclusions about the advisability of installing a solar system can only be drawn after several years of observations and calculations..
Unfortunately, there is little point in relying on solar maps, as local weather conditions can greatly distort the average. Therefore, the first thing to do is to independently compile a report on the intensity of solar radiation in the place where the collectors are installed. Pyranometers are used for measurements; within 5 thousand rubles, you can purchase a budget device with a sufficient set of functions.
Measurements should be taken at different times of the day with a frequency of about a week throughout the year. In the process of measurements, the inclination angle and orientation of the collectors must be taken into account. The resulting data are eventually verified with the statistics of the hydrometeorological center on the percentage of cloudy days per year..
To ensure high efficiency of the solar plant, the most negative scenario should be considered, that is, the longest period with the lowest illumination should be taken as a reference point. Ideally, you can make allowances for the likelihood of even worse weather conditions using meteorological statistics over the past 15-20 years. The obtained data on the incoming solar energy will help to establish the required total area of the absorption field and determine the number of collectors that need to be purchased.
As mentioned, collectors are very rarely used as the main source of heating, they usually play an auxiliary role. But the share of participation can be calculated, it is indicated as a percentage of the total power of the house’s power system or its heat loss. Having received the required number of kilowatts, it is multiplied by the optical efficiency of the absorbers, several coefficients are added – corrections for orientation, tilt, temperature conditions, as well as a safety margin.
According to the “net” value of the generated power, the following is selected:
- the required number of collectors of a certain model and, on average, one backup solar collector per 10-15 in operation;
- piping system with the manufacturer’s recommended throughput and heat resistance;
- circulation group, shut-off valves, other auxiliary devices;
- volume and location of the storage tank. In systems with a daily storage or heat extraction capacity of more than 20 kW, it makes sense to build insulated concrete tanks with a volume of 15–20 m3.
For self-installation and maintenance, it is necessary to draw up a system design, allocate a place for placing auxiliary devices and fix the solar collector on the southern (for the northern hemisphere) roof slope, taking into account the recommendations of the equipment supplier regarding wind loads. Do not forget that by purchasing a full range of equipment from one distributor, you get the opportunity to draw up for free, if not a project of a solar heating system, then at least a list of well-compatible equipment and components.
Do I need a heat pump
One of the main disadvantages of solar heating systems is the high cost. While the technology for the production of flat collectors is well mastered, vacuum absorbers remain expensive, and in certain weather conditions it will only be possible to successfully operate them. But there is another alternative – air-type collectors.
Air-type solar collector
Due to the simpler device, their cost is less, plus there is the possibility of autonomous operation. The efficiency of the air collectors is increased by the installation of a blower fan powered by an integrated solar panel. Due to the accelerated, but proportional to heating, cooling of the channels, return heat losses through the collector are minimized. Power limitation can be achieved by controlling the fan speed or by simply blocking the flow – the air collectors are not afraid of thermal shock, moreover, it is easy to set up natural recirculation.
Lack of air systems in a small degree of heating of the coolant. The heat capacity of the air is less, plus the absorber is almost always heated without focusing. To be able to integrate into the heating system (which is most often necessary due to the impossibility of laying a ventilation duct into a heated room), a heat pump or a split system is really needed.
But air source heat pumps can also be used to increase the efficiency of air conditioning. With them, the circulation rate can be raised to values that are not acceptable in domestic ventilation systems, which gives a 2-3-fold increase in output due to the high temperature difference. At night, the collector will also have a low production rate at the operating temperature range.
The air used as a heat carrier can be dehumidified or replaced with carbon dioxide or another more heat-retaining gas. However, it makes no sense to use heat pumps with a water primary circuit: they are initially designed to operate with a high temperature difference and therefore the power increase is not enough to justify the cost of the installation.
Solar heating system cost
The pleasure of using clean energy comes at a high price, at least for today. To be fair, there is some positive news: over the past five years, the cost of producing flat-plate collectors has fallen by 2-2.5 times, the same can be expected soon from devices with vacuum absorbers.
The cost of flat and vacuum collectors is determined by the volume of production – the value of solar radiation in ideal lighting conditions, that is, the specific power. On average, for 1 kW of flat-type solar collectors, you will have to pay about $ 350-500, and for a complete installation with an external battery – about $ 800-1000. The cost of vacuum solar collectors fluctuates in a higher range – from $ 600 to $ 1000-1200 per complex, depending on the quality of performance, tube material, heat exchanger insulation and other features.
For capacitive collectors, the measurement standard applies in liters of water heated to the highest possible temperature. The amount of generated electricity can be calculated either by the total area of the absorber, or by expressing it through the specific heat capacity of water. Depending on the complexity of the system, the cost varies greatly, the price of one of the examples from the middle market segment reaches $ 1,500 for 300 liters (for 4-5 residents) with a temperature difference of about 50 ° C, which is equivalent to 2.5 kW of specific power.