Developing of a ventilation system for residential houses with solar air heater

Ventilation of buildings in winter period, especially in cold climatic conditions, requires rather big quantity of heat for preheating the ventilation fresh air. Consequently, an average low-income family pays a tangible cost for ventilation of apartments. To provide low cost ventilation of houses it is becoming attractive the use of solar energy for preheating the outside fresh air, before its supplying into the house. For this purpose, cheap solar air heaters are needed. To solve this problem a simple and cheap construction of solar air heater was developed, which is the main part of the ventilation system. The system consists of air heating solar collectors, installed on the roof of the house, warm air distribution ductwork, internal air circulation ducts and air-circulating fan. In summer period, the system is used only during nighttime period for circulating outside colder air through the internal space of the house. Such operation allows cooling internal air and as well the constructions of the house. As a result, the house accumulates nighttime natural cold for using it at daytime cooling purposes. Presentation includes the scheme, explanation of operation and method for design of suggested ventilation system. Analysis proved the energy efficiency and cost effectiveness of the new system.


Introduction
The ventilation of houses in winter period needs initial heating of outside fresh air up to the required inside temperature of a house. The preliminary heating of fresh air demands rather big consumption of energy and fuel. For this reason before supplying of ventilation fresh air into the house, it should be initially heated in special "hot water to air" type heat exchangers.
According to the construction norms and rules the quantity of ventilation outside fresh air depends on sizes of the house and number of habitants. Each habitant of a standard residential house should be provided with g vent. =20 m 3 /h or g vent =0,0056 m 3 /s of fresh air. For example a standard family house, specified for 10 family with 5 person each or totally for 50 habitant of the house requires a quantity of ventilation air that can be calculated by the following equation: where : . vent g = 0.0056 m 3 /s standard norm of fresh air for 1 habitant per second, n hab. = 250 habit. -number of habitants of the house,  air =1.25 kg/m 3density of air.
Substitute of above values in (1) and making calculations will determine the quantity of ventilation air to be supplied into the average residential building per second: For heating the ventilation fresh air from outside t out = -19 o C to inside t ins = 18 o C temperature the heat consumption Q vent will make a value which is calculated by the following equation: (2) where: c air =1.04 kJ/(kg o K) -specific heat of air, t in =18 o C, t out =-19 o C -inside and outside air design temperatures.
Substitute of data in (2) and making appropriate calculations will obtain the quantity of heat Q vent , needed for preheating of ventilation fresh air: For supplying of 67.34kW of thermal energy from gas heater will be needed fuel gas V gas the quantity of which is determined by the following formula: where: Q ventquantity of heat for heating ventilation outside fresh air, kWh b  =0.8 -energy efficiency of heating boiler, Q f = 34700 kJ/m 3heating value of natural gas fuel.
Substitute of obtained values in (3) and making calculations will determine the fuel (gas) consumption V gas , in case of use of a gas boiler makes: where : m=0.5 -gas consumption seasonal averaging coefficient season, Z seas =5500h -duration of winter season.
As in Armenia the cost of 1m 3 of gas makes approximately $0.32, the quantity of natural gas, consumed only for preheating of the ventilation air, will cost at least $7603 per heating season, which is a significant amount for low income families. The consumption of gas fuel can significantly be reduced, if the fuel gas is replaced by solar energy.

Method for calculation and design of void type air solar heater
The existing types of air solar heaters [1], are rather complicated and has low efficiency. Besides, the method for its design is rather rough. For providing efficient solar heating simple and cheap air heaters should be developed and used. For this reason, the authors of this presentation have developed a new simplified and cheap construction of solar air heater, which is presented in fig.1.

Fig.1. New simplified and cheap construction of solar air heater
The suggested solar air heater represents itself a rectangular metallic box made of steel angle bars. The upside frame is covered by 2 mm thick glass. The side walls and the bottom of the box are made of 1.5 mm thick iron sheets. In the inside space of the box nothing is located, it is empty. For this reason, the suggested solar air heater is called void type solar air heater. The void type solar air heater operates in the following way: solar rays penetrate through upside glass cover (3) into the rectangular dark colored box (1) and are absorbed by black internal surface, which is insulated (2) from outside. Outside air enters into the box through inlet duct (4) where contacts to internal hot surface of the box and is heated. The hot air exits from the hot box by the opposite side duct (5) and passes to the ventilation system of the house.
The goal of calculation of the suggested heater is determining the width (b, m), length (l, m) and height (, m) of the hot box for providing required temperature of the air t v.h , o C under given outside temperatures t out , o C and intensity I, W/m 2 of solar radiation.
For describing the mentioned correlation a method of calculation and design has been developed, which is based on the following differential equation of energy balance: Integrating the equation (4) and making simplifications with adoption the conditions F=0; t=t out the following equation is obtained: As follows from the diagrams, the increase of the length l provides higher final temperatures of the air. However, it tends to its limit and stays practically unchanged in case of lengths: l=2m for single glazed, l=3m for double glazed, and l=8m for triple glazed glass covers.
The diagrams of fig.2 show also, that required for ventilation air temperature t = 18 o C is provided under outside temperature t out =-19 o C even in the l =2m long single glazed solar heater. In double and triple glazed heaters having length l =2m the ventilation air under outside temperature t out =-19 o C is heated respectively up to 24 o C and 40 o C, which is a good advantage from energy efficiency point of view. However, the higher costs of double and treble glazed heaters should be taken into account. Thus, it is becoming clear the factor of optimization of the heaters in the solar ventilation projects should be taken into account.
The diagram explains also that under higher outside temperatures t out the final temperatures t of the heated air grows. For example, under t out =8 o C in the 2m long and single glazed heater the final temperature can reach up to 45 o C, and in double and treble glazed heaters up to 52 o C and 68 o C. This means that during most part of heating season the efficiency of the heater will be much higher.
The diagrams of fig.2 are plotted for heaters having fixed parameters =0.05m and ω=0.05m/s. To evaluate the influence of those parameters on the values of final temperatures computational experiments were executed considering that the values of  and ω are variable. The results of calculations made for single and double glazed heaters are represented in fig.3a and fig.3b. From the diagrams is clear that the increase of  and ω conditions growing of air flow through the heater and, therefore, the final temperatures t of air decrease. Consequently, it is better choosing lower values for  and ω. In this case the width b, m of the heater will increase. Therefore, the design of the heater should be made for acceptable temperature and constructive parameters.
For satisfying the stated conditions first, it is important to select the heater's technically acceptable length l, which should not exceed 6-8m. Then, the width b, m of the heater has to be defined for given ventilation airflow using the following fraction: In the last formula the value of the production ω is defined by the values of outside air t out and heated air t temperatures, as well as by selected length l, m of the heater. For this purpose the following formula has to be used: If the width b, m of the heater exceeds the available technical facilities, then double or treble glazed heater should be selected and the same calculation has to be executed until all thermal and constructive conditions are satisfied.