Solar collector

Abstract

A solar collector is made primarily of plastic materials. A wide variety of collector configurations may be made from similar parts, and thus the collector may be adapted to mount in locations where traditional flat panel collectors may not be feasible. The collector may conveniently be used with a nanofluid as the heat transfer fluid, to increase the heat transfer characteristics of the heat transfer fluid. A control system for stagnation remediation, freeze protection, or both may be utilized. For example, when the collector is in danger for stagnation or freezing, water may be circulated through a ground coupled heat exchange loop to cool or heat the collector. Preferably, the stagnation remediation mode does not sacrifice thermal energy previously collected and stored.

1 . A solar energy collector, comprising:
 an elongate plastic receiving tube having an open first end and an open second end, the receiving tube configured to accept a heat transfer fluid into the first end and to carry the heat transfer fluid to and out of the second end;   an elongate clear plastic tubular sheath surrounding the receiving tube, the sheath being of a larger cross section than the receiving tube such that a generally annular air space is formed between the receiving tube and the sheath, the sheath having a front side configured to be disposed toward the sun and a back side opposite the front side; and   a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side of the tubular sheath and configured such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube.   
     
     
         2 . The solar energy collector of  claim 1 , wherein the receiving tube is made of black polyethylene tubing. 
     
     
         3 . The solar energy collector of  claim 1 , wherein the sheath is made of clear polycarbonate tubing. 
     
     
         4 . The solar energy collector of  claim 1 , wherein both the receiving tube and the sheath have circular cross sections. 
     
     
         5 . A solar energy collector, comprising:
 a plurality of elongate plastic receiving tubes each having a respective open first end and a respective open second end, and each of the plurality of receiving tubes being configured to accept a heat transfer fluid into the first end and to carry the heat transfer fluid to and out of the second end;   a plurality of elongate clear plastic tubular sheaths, each sheath surrounding a respective one of the plurality of receiving tubes, each sheath being of a larger cross section than its respective receiving tube such that a generally annular air space is formed between sheath and the receiving tube, each sheath having a front side configured to be disposed toward the sun and a back side opposite the front side, wherein a portion of the interior surface of the sheath at the back side is covered by a reflective coating configured such that when sunlight is directed to the uncoated portion of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly and second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube;   an inlet manifold including an inlet opening for receiving the heat transfer fluid to be heated and a plurality of outlet openings; and   an outlet manifold including a plurality of inlet openings and an outlet opening:   wherein the outlet openings of the inlet manifold are coupled respectively to the first ends of the plurality of receiving tubes, and the inlet openings of the outlet manifold are coupled respectively to the second ends of the plurality of receiving tubes such that each of the plurality of receiving tubes is coupled between one of the plurality of outlet openings of the inlet manifold and one of the plurality of inlet openings of the outlet manifold.   
     
     
         6 . The solar energy collector of  claim 5 , wherein the inlet and outlet manifolds are made of plastic. 
     
     
         7 . The solar energy collector of  claim 5 , wherein the plurality of receiving tubes are disposed parallel to each other to form a rectangular collector unit. 
     
     
         8 . The solar energy collector of  claim 7 , wherein:
 the inlet manifold, the outlet manifold, the plurality of receiving tubes, and the plurality of sheaths are comprised in a first collector unit; and   the solar energy collector comprises one or more additional collector units of like construction to the first collector unit, the inlet manifolds of the one or more additional collector units operatively coupled to the inlet manifold of the first collector unit and the outlet manifolds of the one or more additional collector units operatively coupled to the outlet manifold of the first collector unit.   
     
     
         9 . The solar energy collector of  claim 7 , wherein gaps exist between adjacent members of the plurality of sheaths. 
     
     
         10 . The solar energy collector of  claim 7 , wherein the solar energy collector has an aspect ratio of at least 3:1. 
     
     
         11 . The solar energy collector of  claim 7 , wherein the solar energy collector has an aspect ratio of at least 5:1. 
     
     
         12 . A solar energy collection system, comprising:
 a solar energy collector comprising an elongate plastic receiving tube having an open first end and an open second end, the receiving tube configured to accept a heat transfer fluid into the first end and to carry the heat transfer fluid to and out of the second end, wherein the receiving tube is surrounded by an elongate clear plastic sheath, the sheath being of a larger cross section than the receiving tube such that a generally annular air space is formed between the receiving tube and the sheath, the sheath having a front side configured to be disposed toward the sun and a back side opposite the front side and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube; and   a supply of the heat transfer fluid to be heated by the solar energy collector.   
     
     
         13 . The solar energy collection system of  claim 12 , wherein:
 the solar energy collector comprises a plurality of elongate plastic receiving tubes, each surrounded by a respective elongate clear plastic sheath;   the plurality of sheaths and receiving tubes are arranged in parallel such that the solar energy collector is rectangular; and   the solar energy collector comprises an inlet manifold and an outlet manifold to direct the heat transfer fluid through the parallel receiving tubes.   
     
     
         14 . The solar energy collection system of  claim 12 , wherein the heat transfer fluid is water comprising nanoparticles. 
     
     
         15 . The solar energy collection system of  claim 14 , further comprising an ion generator to generate the nanoparticles. 
     
     
         16 . The solar energy collection system of  claim 12 , further comprising:
 a tank for holding the supply of heat transfer fluid;   a supply pipe for carrying heat transfer fluid from the tank to the solar energy collector;   a return pipe for carrying the heat transfer fluid from the solar energy collector to the tank;   a circulation pump for circulating the heat transfer fluid between the solar energy collector and the tank through the supply pile and the return pipe; and   a controller that controls the operation of the circulation pump based at least in part on the temperature of the heat transfer fluid in the tank and the temperature of the solar energy collector.   
     
     
         17 . The solar energy collection system of  claim 16 , wherein the controller is configured to determine when the solar energy collector reaches a stagnation condition and in response to the determination, enter a stagnation remediation mode. 
     
     
         18 . The solar energy collection system of  claim 17 , further comprising a source of cooling fluid, wherein during the stagnation remediation mode, the cooling fluid is circulated through the solar energy collector without passing through the tank. 
     
     
         19 . The solar energy collection system of  claim 17 , further comprising a ground coupled heat exchanger, wherein in the stagnation remediation mode, the cooling fluid is circulated through the solar energy collector and the ground coupled heat exchanger without passing through the tank. 
     
     
         20 . The solar energy collection system of  claim 19 , wherein the ground coupled heat exchanger comprises a piping loop connected between the supply pipe and the return pipe, and wherein the system further comprises a set of valves operated by the controller to isolate the tank during the stagnation remediation mode. 
     
     
         21 . The solar energy collection system of  claim 16 , further comprising a photovoltaic panel that supplies power to operate the controller and the circulation pump. 
     
     
         22 . The solar energy collection system of  claim 16 , further comprising a root zone heating loop, wherein the system circulates the heat transfer fluid through the root zone heating loop to heat the root zone of plants. 
     
     
         23 . A method of collecting solar energy, the method comprising:
 providing a solar energy collector comprising one or more elongate plastic receiving tubes, each of the one or more receiving tubes having a respective open first end and a respective open second end, and each of the one or more receiving tubes being configured to accept a heat transfer fluid into the first end and to carry the heat transfer fluid to and out of the second end, each of the one or more receiving tubes being surrounded by a respective elongate clear plastic sheath, each respective sheath being of a larger cross section than its respective receiving tube such that a generally annular air space is formed between the receiving tube and the sheath, the sheath having a front side configured to be disposed toward the sun and a back side opposite the front side and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube;   installing the solar energy collector in a location that receives sunlight; and   passing a heat transfer fluid through the solar energy collector to be heated by the sunlight.   
     
     
         24 . The method of  claim 23 , wherein the solar energy collector has an aspect ratio of at least 3:1, and wherein installing the solar energy collector in a location that receives sunlight comprises installing the solar energy collector in a location that cannot accommodate a collector of equal area having a significantly smaller aspect ratio. 
     
     
         25 . The method of  claim 23 , wherein the solar energy collector has an aspect ratio of at least 5:1, and wherein installing the solar energy collector in a location that receives sunlight comprises installing the solar energy collector in a location that cannot accommodate a collector of equal area having a significantly smaller aspect ratio. 
     
     
         26 . The solar energy collector of  claim 1 , further comprising a plurality of insulating spacers disposed in the generally annular air space, for maintaining the spacing of the sheath from the receiving tube. 
     
     
         27 . The solar energy collector of  claim 1 , wherein the solar energy collector has only one receiving tube. 
     
     
         28 . The solar energy collection system of  claim 18 , further comprising:
 a first normally-open valve in the supply pipe and a second normally-open valve in the return pipe, wherein closing the first and second normally-open valves prevents flow of the heat transfer fluid into between the tank and the solar energy collector;   a cooling loop containing the cooling fluid, the cooling loop joining the supply pipe and the return pipe; and   a cooling pump;   wherein when the first and second normally-open valves are closed, the cooling fluid can circulate through the cooling loop and the solar energy collector via the supply pipe and the return pipe, driven by the cooling pump.