46352 State Hwy 329 Morris, MN 56267 Phone: (320) 589-1711

Website: wcroc.cfans.umn.edu

Solar Water Heating

In the United States, heating water accounts for about 17% of the total energy consumed in residential buildings. One of the most cost-effective ways to include renewable technologies in a build-ing is by incorporating solar hot water. A typical residential solar water-heating system reduces the need for conventional water

heating by about two-thirds. It minimizes the expense of electricity or fossil fuels to heat the water and reduces the associated environmental impacts. However, solar water heating cannot entirely replace conventional water heaters.

System Design and Costs

As a general rule, the system should be designed to provide close to 100% of the summer hot water load, resulting in 45% to 80% of the annual hot water load being met. Increased cloud cover and reduced solar insolation can result in a solar contribution as little as 20% in the winter. If the system is designed to provide more hot water in the winter, a method of using or dumping the excess heat in the summer must be included to prevent overheating.

According to the Solar Rating & Certification Corporation (SRCC), average residential hot water usage requires 41,045 Btu per day which is equivalent to about 12 kWh per day. Knowing a system’s solar fraction (the portion of the hot water heating load provided by solar energy) allows the estimation of savings in operating costs.

Annual Gas Savings = 365 X (.41045 / 0.6*) X Solar Fraction X Fuel Cost (therm)

Annual Electricity Savings = 365 X (12.03 / 0.9*) X Solar Fraction X Fuel Cost (kWh)

*SRCC standard Energy Factor for auxiliary water heater (www.solar-rating.org)

How It Works

Most solar water-heating systems for buildings consist of a solar collector, a storage tank, and a control system. There are two types of solar thermal panels: flat plate collectors (FPC) and evacuated tube collectors (ETC). The most common collector used in solar hot water systems is the flat-plate collector, but evacuated tube collectors are more efficient than FPC’s at the higher tempera-ture differences expected in winter and as such are gaining in popularity as their price comes down.

Solar water heaters use the sun to heat either the household water (direct system) or a heat-transfer fluid (indirect system) in the collector. The heated water is then held in a storage tank with a conventional system providing additional heating as necessary. The tank can be a modified standard water heater or a separate heat exchanger tank that feeds into a standard water heater. Solar water heating systems can be either active or passive, but the most common are active systems. Passive systems do not use an electric pump, rely-ing instead on gravity or convection for circulation. Active, indirect systems are the most efficient and the most expensive but are best suited to a cold climate.

Flat Plate Collector

Evacuated Tube Collector

Source: REN21 2016, Renewables Global Status

46352 State Hwy 329 Morris, MN 56267 Phone: (320) 589-1711

Website: wcroc.cfans.umn.edu

Solar Water Heating System at the WCROC

The U of M Initiative for Renewable Energy and the Environment (IREE) and the State provided funds to establish a small scale renewable energy test bed at the WCROC in Morris. One of the systems to be tested will provide energy for hot water in the building.

System Architecture

The system uses two flat plate solar collectors connected by a closed loop, filled with a glycol/water heat transfer fluid, to a Trendsetter heat exchanger tank. The output of this tank is connected to the input of the electric back-up water heater and then to the building hot water load. There is a recirculation pump which continuously pumps water through the building plumbing system to maintain hot water at each tap. When hot water is used, it is re-placed by the cold water supply to the heat exchanger tank.

When hot water is used in the building, the cold water is pre-heated as it courses through the heat exchanger immersed in the solar water stored in the Trendsetter tank. If the solar panels are providing enough heat, hot water will simply pass from the Trendsetter tank through the electric back-up heater without engaging it. If the water temperature from the Trendsetter drops below the back-up heater’s set point, the pre-heated water will reduce the energy normally needed to raise the water temperature entering the water heater. For example, if the cold water entering the water heater is 60°F, the water heater will require energy to raise the incoming water from 60°F to 120°F (8,616 Btu per 100 gallons). If the entering water temperature were raised to 100°F, the same 100 gallons of hot water would consume only 2/3 the energy (2,878 Btu).

System Facts

The building is about 13,700 ft2

64 ft2 (6 m2) of flat plate collector area provided by Solar Skies, model SS-32

Trendsetter TS-100 thermal tank, 105 gallon capacity

Back-up water heater is 40 gallon Marathon electric unit, model MR40245B

Peak power output was about 2100 Btu/h (0.6 kW) in August 2015 and 4100 Btu/h (1.2 kW) in March 2016

Total energy collected from June 2015 through May 2016 was 541.5 kBtu (158.7 kWh)

Heat transfer fluid is a 50/50 mix of water and propylene glycol

Solar Hot Water System

WCROC Hot Water System